FORTNIGHTLY REVIEW.

No. XVI. NEW SERIES.—APRIL 1, 1868.

MR. DARWIN'S HYPOTHESES.

I.

"THE ORIGIN OF SPECIES" made an epoch. The product of an immense series of tentative gropings, it formed the turning-point of an entirely new series: concentrating as in a focus the many isolated rays emitted by speculative ingenuity to illuminate the diversified community of organic life, it propounded an hypothesis surpassing all its predecessors in its congruity with verifiable facts, and in its wide-reaching embrace. Because it was the product of long-continued though baffled research, and thereby gave articulate expression to the thought which had been inarticulate in many minds,1 its influence rapidly became European; because it was both old in purpose and novel in conception, it agitated the schools with a revolutionary ferment. No work of our time has been so general in its influence. This extent of influence is less due to the fact of its being a masterly work, enriching Science with a great discovery, than to the fact of its being a work which at once clashed against and chimed with the two great conceptions of the world that have long ruled, and still rule, the minds of Europe. One side recognised a powerful enemy, the other a mighty champion. It was immediately evident that the question of the "Origin of Species" derived its significance from the deeper question which loomed behind it. What is that question?

If we trace the history of opinion from the dawn of Science in Greece through all succeeding epochs, we shall observe many constantly-reappearing indications of what may be called a premonitory feeling rather than a distinct vision of the truth that all the varied manifestations of Life are but the flowers from a common root,—

that all the complex forms have been evolved from pre-existing simpler forms. To the early speculators such a feeling was enough. Knowing little of the intellectual needs of our time, they were careless of precision, indifferent to proof. But when such a point of view had once been adopted, it revealed consequences irreconcilable with the reigning doctrines; and was therefore challenged sharply by the defenders of those doctrines, and called upon to produce its evidence, furnish proofs. Unhappily, it had little evidence, no proof. The scientific intellect found no difficulty in making what was offered as evidence appear quite inadequate. The more precision a few ingenious advocates endeavoured to give to their arguments, the more glaringly absurd the speculation seemed. To men largely acquainted with the phenomena of organic life, and trained in the habits of inductive inquiry, there was something repulsive in the crude disregard of evidence exhibited in such theories as those of De Maillet and Robinet.1 A certain discredit was thrown on the hypothesis by the very means taken to recommend it. So long as it remained a vague general notion, it was unassailable, or at least unrefutable; but on descending into the region of verification, it presented a meagre aspect.

Nevertheless, it survived opposition, ridicule, refutation. In the face of evidence, in the face of ridicule, in the face of orthodoxy very indignant, this idea of the evolution of complex forms from simpler forms persisted; and the reason of this persistence is that the idea harmonises with one general conception of the world—(Weltanschauung, as the Germans say)—which has been called the Monistic because it reduces all phenomena to community, and all knowledge to unity. This conception, under its various forms of Pantheism, Idealism, Materialism, Positivism, is irreconcilable with the rival, or Dualistic, conception, which in phenomena separates and opposes Force and Matter, Life and Body, and which in knowledge destroys unity by its opposition of physical and final causes. The history of thought is filled with the struggle between these two general conceptions. Slightly varying Schlegel's dictum, "Every man is born either a Platonist or an Aristotelian," I think it may be said that every man is somewhat by his training, and still more by his organisation, predisposed towards the Monistic or the Dualistic conception, a predisposition which renders it easier for him to feel the force of the arguments on one side than on the other; and that, in consequence of this native bias, we may generally predict what will be his views in Religion, Philosophy, and Art—to a great extent even in Science. Be this as it may, there can be little doubt that the acceptance or the rejection of Darwinism has, in the vast

(1) Robinet, "De la Nature," Amst. 1766. (I gave an analysis of this book in Fraser's Magazine, Nov., 1857.)

majority of cases, been wholly determined by the Monistic or Dualistic attitude of the mind.

And this explains, what would otherwise be inexplicable, the surprising fervour and facility with which men wholly incompetent appreciate the evidence for or against Natural Selection have adopted or "refuted" it. Elementary ignorance of Biology has not deterred them from pronouncing very confidently on this question, which involves all the principles of Biology; and biologists with him scorn have asked whether men would attack on astronomical, physical, or chemical hypothesis with no better equipment. Why not? They feel themselves competent to decide the question from higher grounds. Profoundly convinced of the truth of their general conception of the world, they conclude every hypothesis to be true or false, according as it chimes with, or clashes against, that conception. Starting from this point, each party throws its whole energy into collecting (oftener snatching at rather than collecting) evidence and arguments, flavoured with ridicule and rhetoric, for or against the hypothesis. Only desirous of vindicating a foregone conclusion, they rarely attempt a meditative and dispassionate survey of the evidence.

So it has been, so it will long continue. The Development hypothesis is an inevitable deduction from the Monistic conception of the world; and will continue to be the battle-ground of contending schools until the opposition between Monism and Dualism ceases. for myself, believing in the ultimate triumph of the former, I look on the Development Hypothesis as one of the great influences which will by its acceptance, in conjunction with the spread of scientific culture, hasten that triumph, teaching us, to use Goethe's words,—

But it is one thing to hold firmly to the Development Hypothesis, another thing to accept Natural Selection as the last word on that subject. Darwinism is undoubtedly a better explanation than any of its forerunners; but it will probably give place to some successor, as the hypotheses of Geoffroy St. Hilaire, Meckel, Lamarck, Bonnet, and Robinet gave place to it. Meanwhile, it is the best hypothesis at present before the world, and has converted many naturalists who before were sceptical. For I should convey a false impression by what was said just now if I did not add that many biologists whose conception of the world was purely Monistic rejected with scorn the explanations of Lamarck and others as to the origin of species; and although the luminous suggestion of Natural Selection has converted some of these, there still remain many unconvinced. The immense superiority of Darwinism is that it not

only puts forward as the cause of all variation a law which demonstrably the cause of much variation, but includes also the vera causœ suggested by Lamarck and Meckel. The law of Nature Selection may indeed be said to be only a larger and more philosophic view of the law of Adaptation which Lamarck had imperfectly conceived. We must not, however, underrate the singular importance of Lamarck's hypothesis in calling attention to the modifiability of structure through modifications of adaptation; though he was led into exaggerations by a one-sided view, which made him attribute too great an influence to one set of external conditions. Naturalists before his time had been wont to consider the Organism apart from the Medium in which it existed; he clearly saw that vital phenomena depended on the relation of the two; but in his hypothesis he sacrificed the one factor somewhat to the other; he paid too little regard to the Organism and its laws of development. Meckel captivated attention by the striking illustrations from embryology1 in proof of Kielmeyer's position that all existing organisms are modifications of a single type, all the stages of the lower types being indicated in the successive transformations of an embryo of the highest type; but a rigorous criticism showed that in this form the hypothesis was not tenable.2 The hypothesis put forth in the "Vestiges," though it had the merit of connecting the organic evolution with the cosmical evolution, uniting the hypotheses of Lamarck and Meckel with the nebular hypothesis of Kant and Laplace, laboured under the general disadvantage of reposing on two principles which only a metaphysician could accept as rerœ causœ. One of these was the conception of a pre-existent Plan, according to which organisms were suppose to have been formed (the ύστερον λρόλρον fallacy); the other the conception of Time as a factor apart from all the conditions.3 We need discuss neither here. But the helplessness of such metaphysical explanations is well exhibited in the case of rudimentary organs- perhaps the strongest case against final causes—which appear to the author of the "Vestiges" as "harmless peculiarities of development and interesting evidences of the manner in which the Divine Author has been pleased to work."4

Minds unconvinced by all such attempts were at once subdued by the principle of Natural Selection, involving as it did, on the one hand, the incontestable Struggle for Existence, and on the other, the known laws of Adaptation and Hereditary Transmission. There still

(4) To a similar effect Agassiz, who asks, "Does not the existence of a rudimentary eye in the blindfish show that these animals, like all others, were created with them peculiarities by the fiat of the Almighty, and that this rudiment of eye was left them a remembrance of the general plan of structure of the great type to which they belong?"— "Essay on Classification," 1859, p. 20.

remain philosophers and theologians who have an "intuition" of its falsehood, and naturalists who fail to see how it clears up a mass of difficulties; the legitimate opposition of these adversaries will go far towards a furtherance of the final solution. Meanwhile adherents regard Mr. Darwin's work as crowning the labours of a century. There is, indeed, a curious coincidence of dates noticed by Haeckel.1 Exactly one hundred years, he reminds us, elapsed between the "Theoria Generationis" of Wolff (1759), which by the doctrine of Epigenesis laid the foundation-stone of the theory of Development, and the "Origin of Species" (1859), which supplied the coping-stone. Nor does the coincidence of dates end here. For half a century, he says, the doctrine of Wolff remained almost dormant, till, in 1806, it was made the common property of the scientific world by Oken's exposition of the mode in which the intestinal canal was developed (which was mainly a re-statement of the exposition given by Wolff in his Memoir "De Formatione Intestinorum," 1766). In like manner the theory of Descent, which Lamarck produced in 1809, had to wait fifty years before it received its scientific consecration in the "Origin of Species."

It would be easier to write a volume on this vast subject than a satisfactory essay; and as I cannot indulge my inclinations with writing a volume, I only propose to discuss two or three of the topics directly involved, especially to answer the objections which are regarded as the most serious, namely: 1. Why have Species not varied during the four thousand years of which we have record? 2. Why are domesticated animals, when suffered to run wild, always found returning to the primitive wild type? 3. Why are not new species constantly produced, and why are not the intermediate forms discoverable? Having answered these questions, I shall have something to say respecting Mr. Darwin's hypothesis of Natural Selection as the determining cause of specific forms, and respecting his hypothesis of Pangenesis as the determining cause of inherited forms.

II.

What is meant by Species? A man unversed in and unperplexed by the dicta of naturalists would simply answer: A kind of plant or animal. But on turning to the authorities for a more precise definition, such as would enable him to particularise the kind, and describe the characters by which it could be identified, he would find himself in presence of strange contradictions. A little experience

(1) Haeckel, "Generelle Morphologie der Organismen," 1866, ii. p. 8. Many readers will be grateful for having their attention directed to this work, one of the most instructive contributions to the philosophy of Biology which has appeared in our time. It will assuredly give great offence to many by the way it rides rough-shod over dogmas theological and biological, and by its wide-sweeping scorn of systematists and specialists; but it is rich in special knowledge and suggestive ideas. Mr. Darwin has reason to be proud of his disciple.

would disclose that even the most authoritative naturalists had one rule to be followed in theory, another and very different rule to be followed in practice. On this point we may say with Müller,1 that as in a Christian country there is a Catechism every one repeats and no one considers himself bound to follow, or expects others to follow, so in Zoology there are dogmas which one's practice denies. Among a hundred writers who feel upon to preface their treatises with a confession of faith, ninety-nine begin with a grave exposition of the rule that a natural system not be founded on any one character, but on all the character must take into consideration the whole organisation, and not estimate characters as of equal value, but according to their physiological rank, &c. But on passing to the actual work of classification attempting to range the animals into Species, Genera, and Fa there is probably not one of the ninety-nine who thinks of applying these philosophic rules. Thus Agassiz follows Cuvier in making Radiata a branch of the animal kingdom, although nobody has any idea what may be the importance in the life of the animal this radiate structure may have, and in spite of the well-fact that the radiated echinoderms issue from bilateral larvæ. fishes are divided into Ctenoid and Cycloid, according as the margins of their scales are toothed or rounded—a detail which must infinitesimal importance in the life of the animal. Sometimes and animals are classed as different Species when they differ only in colour,2 in size, in shape, in habits, or instincts; at other times they differ widely in any or all of these characters, they are together, and are called Varieties.

Not only does the practice contradict the rules, the rules selves are contradictory, and eminently capricious. Linnæus species thus: "Species tot sunt diversæ quot diversæ formæ ab initio sunt creatæ." But who shall say what were the forms originally created? And when Cuvier appeals to the bond of parentage, defining species as "la réunion des individus descendant l'un l'autre et des parents communs, et de ceux qui leur resse autant qu'ils se ressemblent entre eux," the rule would be excellent if we were always in possession of all the genealogical data; point of fact, even with regard to domesticated animals, we always trace this family bond, and with regard to wild animal wholly an assumption. An attempt is made to prove the relat by the evidence of indefinite fertility; and this character is on

regarded as decisive of species, only those plants and animals being held to be of the same species which are indefinitely fertile with each other. So much stress is laid on this point that I would willingly accumulate pages of evidence against it, if space permitted; but three considerations must suffice. First, it is not true, and Mr. Darwin has proved it not to be true, that any species is indefinitely fertile where the bond of kinship is closest; breeding in and in always terminating in sterility. Secondly, the generative system is so readily affected by slight changes in the conditions of life, that animals undeniably of the same blood are sterile under those conditions. Thirdly, animals sterile with some members of the species are fertile with others, and fertile with members of different species. "It is a great law of nature," says Mr. Darwin, in his latest work, "that all organic beings profit from an occasional cross with individuals not closely related to them in blood; and that, on the other hand, long-continued close interbreeding is injurious."1 Hence it is utterly fallacious to argue from fertility. Moreover, when a species is known to us only through one individual, how are we to determine whether it is a Species or Variety? Obviously we can only say, Here is a form which differs from all other known forms; and it is on this difference that we assign it a place in our system. By its resemblances we bring it under one group; by its distinctive traits we isolate it in that group. Thus recurs the unscientific definition: "Species means a kind of plant or animal." While the chemist can furnish a precise and unvarying definition of chemical species, the naturalist can only furnish a vague and varying definition. The kind of resemblance and difference which one naturalist regards as specific, another holds to be generic, and a third to be simply the mark of a variety.

Very important is it to bear in mind that Species is a subjective creation having no objective existence: it is an idea, not a thing; a systematic artifice, not a living entity. This is clearly enough expressed in the favourite definition: "Species is a succession of individuals capable of reproducing themselves;" but when naturalists argue about fixity of species, they mostly overlook this conception of a succession and its implications, to replace it by a conception of an abstract form, an unvarying entity which is independent of the individuals. On several occasions I have called attention to the lingering remnant of Scholasticism cherished in the arguments de-

(1) He mentions the case of a sow who would not breed at all to her sire, but bred at once to a stranger in blood. Another sow, the product of close interbreeding for three generations, when paired with her own uncle (known to be productive with other sows), produced a litter of only six, and another litter of only five weak pigs; but paired with a boar of a small black breed (which produced seven swine with a sow of his own breed), she who had been so unproductive with her uncle, yielded twenty-one, and in a second litter eighteen pigs.

fending the fixity of species. In the early days of speculation, when it was a first principle that what we know as General Terms had corresponding Objects existing in the external world as distinct realities, and not simply as relations, the belief in a thing Species was rational enough; when philosophers believed that over and above the numberless individual animals they saw around them, there existed an Animal which they did not see, but which was the norm and pattern for all individuals—when they held that over and above the good and bad actions committed by them and their fellows, there existed an Immutable Virtue and an indestructible Evil—when, in short, they held the theory of Ideas, they could have no grounds for suspecting the reality of Species. But it is otherwise in our day. Platonists are rare, and Scholasticism is a scoff. Nevertheless unconscious disciples argue about the fixity of Species as if Species were a thing that could be mutable or immutable. They would deny the charge, no doubt; they are not sufficiently clear on the point to see their real position.

Regarded objectively, what place is held by Species? In certain fundamental traits, all plants and all animals have a community, and on this is founded the first division of the Organic and Inorganic. The next step is to divide Plants from Animals. From the microscopic formless dab of jelly which constitutes the Amœba, up to the marvellously complex structure which we name Man, there is, underlying all diversities, a community on which we found the group Animal. In classifying these diversities we establish groups to which different names are affixed, as indications of the degrees of unlikeness. When the animals differ but slightly, we group them as Varieties; when they differ more, as Species; when they are still more different, as Genera; and so on through Families, Orders, Classes, Sub-kingdoms. That these resemblances and diversities exist objectively—that is to say, that the corresponding phenomena are thus related—is indisputable; and it is therefore not only true, but a truism, to affirm that the Names by which we designate them have a fixed meaning; but it is not true, it is a falsism, to assert that these relations are immutable, being, as they are, the relations of variable individuals.

We should think it very irrational to insist that while bank-notes, shillings, sixpences, and pence were conventional monetary standards, sovereigns were something more than conventional, and had a monetary reality denied to other moneys. It is not less irrational to insist that while the wider divisions of genera, orders, classes, and the narrower divisions of varieties are conventional, the intermediate divisions (species) are not conventional, but real.

I will cite but four writers where it would be easy to cite forty. Buffon says: "Les espèces sont les seuls êtres de la nature. Les

individus sont les ombres dont l'espèce est le corps."1 Cuvier declares that Classes, Orders, and Genera are abstractions, "et rien de pareil n'existe dans la nature;" but instead of logically extending this to the group of slighter differences, he maintains that Species is not an abstraction.2 Flourens, his disciple, says: "Les espèces sont les formes primitives de la nature. Les individus n'en sont que des représentations, des copies."3 To conclude with Johannes Müller: "The species is a living form represented by individual beings, which reappears in the product of generation with certain invariable characters."4

Unless men held Species to have an existence apart from individuals, the question of fixity would have no sense, because the real question is, Are individuals variable? If they are, their relations to each other must vary, and it is their relations which we designate in the terms Species and Genus. That animals do vary is indisputable, undisputed. And here arises the further question: Are these variations only possible within certain ascertained limits, or are the variations indefinite? The majority of naturalists answer that the limits are ascertained, and the term Species corresponds with such limits. Their opponents, at least the more philosophical of them, while admitting that no individual organism can be greatly modified (and it is therefore correct to say of an individual that there are narrow limits of possible variation), assert that the small variations of each individual will so accumulate in the course of numerous successive generations as to transcend all specific limits, and in effect become indefinite. The divergence which is inappreciable at the apex of an acute-angled triangle becomes gradually greater, and at the base may be enormous.

It is not only a surprising simplification of the problem when we thus set aside the metaphysical figment of Species, and direct our attention solely to the facts of variation, and the accumulation of variations through inheritance; but the problem which is thus simplified is also brought from the region of Theology and Metaphysics into the region of Science: it has come within the range of Verification. How much metaphysical and theological misdirection has hitherto confused this subject may be seen in the disguised form of the scholastic conception which moderns have adopted; for I should be doing naturalists an injustice if I allowed the inference to pass that they adopt the crude notion of Species as an objective reality, which their language and arguments imply. It comes to them under two guises and disguises. One is that of the "creative fiat;" the other, and more reasoned hypothesis, is that of "creative plan."

According to the first, plants and animals had their forms ordained for them at the moment of their creation, and these forms are unchangeable. According to the second, the organic world is part of a general scheme, in which each Species represents an Idea in the Divine Mind, and must be taken as an item in a Plan conceived from the first in all its details, although realised in successive epochs. Each Type was impressed once for all on each group; however the individuals in each group may vary among themselves, the Type is unvarying, and constantly effaces the variations of individuals.

III.

The first impulse of a scientific scepticism is to inquire by what means philosophers have acquired this precise knowledge of the Ideas existing in the Divine Mind; very enviable knowledge, but needing some guarantee of its genuineness. If it was gained from the study of Nature, then it must be amenable to all the canons of scientific research; and these assure us that the utmost to be learned in such a study is the persistence of Types,—of their pre-existence nothing whatever can be rigorously ascertained; and these canons further assure us that the persistence of a type is necessarily limited to the persistence of its concurrent conditions. Any hypothesis which starts from an à priori construction of creative fiat, or creative plan, must first justify its origin. In science an explanation is the reduction of phenomena to a series of known conditions, thus bringing what was unknown within the circle of the known. But of creative fiats we can know nothing; we may infer them; and the validity of our inference has to be tested by that very process which constitutes a scientific explanation. To infer that Species were Ideas in the Divine Mind is on a par with the inference once firmly accepted, that anomalies and monstrosities were "freaks of Nature," and the work of demons; or that other inference of fountains and trees being animated with Naiads and Hamadryads. Now that we have learned something of the process of organic development, we have learned that anomalous forms are deviations in the line of growth, due to arrest or excess, and are neither effects of God's wrath nor of Satan's malice.1

The hypothesis of creative fiats begs the question, and explains nothing. It is an hypothesis burdened with the double disadvantage

of being incapable of proof, and incompetent to explain: incapable of proof, for no one can ascertain what was or was not "ordained,"—we can only ascertain what is the order of phenomena within our ken; incompetent to explain, for whenever a variation arises, the only resource is to affirm that this variation also was ordained. Andreas Wagner boldly sought this refuge, affirming (as quoted by Haeckel) that the conception of Species was not applicable to domesticated plants and animals, because they were created variable in order to subserve the purposes of man. It is the peculiarity of this kind of philosophising that its conclusions cannot be refuted because they do not admit of proof. There is always an escape from every objection through some easy supposition invented for the nonce. You think you disprove the notion of an invariable Plan by showing instances of variation? Your objection is set aside by the remark that the variations were also planned.1 You observe that unhealthy organisms transmit their morbid states, and you are assured that Nature "revient par des voies détournées sur la rigueur de ses décrets," as if Nature were full of pity, and relented on the pathway of destruction. An easy phrase eludes all argument. "A chaque type spécifique," says a recent advocate, "on peut rattacher des formes secondaries dérivées, produites par les influences de milieu; si l'on on, méconnaît l'origine, on sera conduit à les considérer comme espèces égitimes, tandis qu'elles sont seulement l'expression de la flexibilité organique."2 If those who maintain the variability of Species are to save their illustrations disposed of by this simple process of rebaptism, it is clear that all argument becomes idle; when "organic flexibility" has any meaning given to it other than specific variability, Language once more proves its services to Metaphysics.

The hypothesis of creative fiats having ordained the existence of species is an evasion of the question, not an answer to it. Moreover, its limitations are strangely unwarrantable. Thus it assumes the remarkable uniformity in the number of segments recognisable in crustacea and insects under the amazing varieties of their forms to be due to conformity with Plan. And as comparative anatomists point out the existence of these twenty segments, even when they are so fused as to present little or no segmentation to the uninstructed eye, the argument seems weighty. But when Nature shows deviations from this Plan, in articulated animals having fewer than twenty segments, or more than twenty, the argument is proved to be inconsistent. "Why," asks Mr. Spencer, "if the skeleton of each species was separately contrived, was this bony mass (the sacrum) made by soldering together a number of vertebræ like those forming

the rest of the column, instead of being made of one simple piece ?" The answer is, that the sacrum is made of segments in conformity with the vertebrate Plan; but Mr. Spencer then asks, "Why does the number of sacral vertebræ vary within the same order of birds? Why, too, should the development of the sacrum be by the round-about process of first forming its separate constituents, and then destroying their separateness?"1 Nor does the contradiction of the hypothesis end here; it assumes that Genera and Species were produced by direct exercise of a Creative Will, whereas Varieties and Races were produced by the operation of natural laws. Such a separation of agencies is unphilosophic; and if we avoid it by the acknowledgment of every individual plant and animal being the product of a creative fiat, then indeed we get rid of the Dualistic conception of Nature, but the difference between the hypothesis of Creation and the hypothesis of Evolution becomes only a difference of terms.

I have endeavoured elsewhere2 to expose the fallacy involved in the notion of Plan or Type as anything more than a subjective concept a nexus we discover in evolved forms, and which we, by natural infirmity, imagine to have been the nisus of those forms— resultant which we imagine to be a principle. To that discussion I must refer, not having space now at command to treat of its bearing on Species. If Type means the correlation of parts which remains constant under all diversities among those parts—the Vertebrate Type, for example, being that correlation of parts which is found in fishes, reptiles, birds, marsupials, and mammals, so that whenever the same parts are found in different animals, the connections of such parts are the same—there are obvious advantages in our being able to use this shorthand phrase; but there are no advantages and many dangers in using the phrase as if it meant that before vertebrate structures existed, a Type existed according to which they were formed.

It is possible that the hypothesis of Natural Selection, which Mr. Darwin opposes to that of creative fiat and fixity of Plan, may be an imperfect explanation, but at any rate it has the immense merit of bringing the question within the region of Research. If it leaves many difficulties unexplained, the rival hypothesis explains none. Some of these we may have to consider hereafter; at present we have to see what its opponents regard as insuperable difficulties.

"If Species are variable, why have they not varied?" This is the objection most frequently urged. Our answer simply is, that as animals have varied, which is all that the hypothesis requires. Were it not for the unconscious influence of the belief in Species as an

(1) Spencer, "Principles of Biology," 1864, i. 383.

(2) In the Prolegomena to the "History of Philosophy," 3rd ed. 1867, p. lxxxv.

entity or as an unchangeable fiat, no one would have been misled by the facts which have misled even philosophic minds. It is only necessary to replace the horse before the cart, only necessary to recognise that the Type (or arrangement of parts) is the result of concurrent conditions, not the cause of their concurrence, to perceive the real value of the alleged objection. We are referred to the testimony of paintings and sculpture some four thousand years old as evidence that several well-known Species and even well-marked. Races of animals and men have not changed. Nimrod hunted with horses and dogs which might be claimed as ancestors by the horses and dogs at Melton Mowbray. Semiramis and Rhamses were served by negroes in every respect similar to those who were toiling amid the sugar-canes of Alabama when President Lincoln decreed their emancipation. The fact is certain. What does it imply? According to the advocates of fixity, it implies that Species cannot be changed. If during four thousand years no change has taken place, why assume that there is an inherent tendency to change? This argument is the cheval de bataille of the Cuvier school, but it turns out on close inspection to be a spavined, broken-winded Rosinante.

In the first place, the testimony proves too much, for it proves that Races are as unchangeable as Species. Now Races, according to all naturalists, are not special creations, but are variations which have become permanent; and as no one holds that particular Types were created for all the variations (that being, indeed, a contradiction in terms), but all hold that Races are the result of modifications impressed on the original Type, the fact of such modifications remaining unchanged during four thousand years entirely robs the testimony of its argumentative value when applied to Species.

In the second place, it may be paradoxical, but it is strictly true,1 that the fact of particular species having remained unaltered during four thousand years does not add the slightest weight to the evidence in favour of the fixity of Species. Four thousand or forty thousand prove no more than four. You would not suppose that I had strengthened my case if, instead of contenting myself with stating reasons once, I repeated those same reasons during forty successive pages; you would remind me that iteration was not cumulation, and that no force could be given to my fortieth assertion which was absent from my first. Why then ask me to accept the repetition of the same fact four thousand times over as an increase of evidence? It is a fact that Like produces Like, that dogs resemble dogs, and do not resemble buffaloes; this fact is deepened in our conviction by the unvarying evidence we see around us, and is guaranteed by the philosophical axiom that "like causes produce like effects;" but when once such a conception is formed, it can gain no fresh strength

from any particular instance. If we believe that crows are now, we do not hold our belief more firmly when we are show crows were black four thousand years ago. In like manner, an admitted fact that individuals always produce individuals resembling themselves, it is not a whit more surprising that the of Victoria should resemble the dogs of Semiramis than that should resemble their parents; the chain of four thousand years is made up of many links, each link being a repetition of the So long as a single pair of dogs resembling each other unite, so long will there be specimens of that species, simply because the chain inherit the characteristics of their parents. So long as negroes marry with negroes, and Jews with Jews, so long will there perpetuation of the negro and Jewish types; but the tenth generation adds nothing to the evidence of the first, nor the ten thousand to the tenth.

All that the fact implies is that during four thousand years has been a concurrence of conditions which has been sufficiently form to preserve the descendants of Species and Races from alter. It is far from proving that simultaneously with this uniformity has been no diversity capable of producing new Races and new Species indeed, a slight consideration suffices to convince us that such diversity has existed, and that side by side with the persistent form forms have arisen. The testimony of Egyptian tombs is value far as it reaches, but naturalists need only wander beyond precincts of those tombs to find forms that have altered beside that remain unaltered. Thus let us suppose an Egyptian king to had one hundred dogs all of them staghounds, and no other form of dog to have existed at that time. These staghounds would transmit to their offspring all their specific characters. But however resemble each other, they always present individual different size, colour, strength, intelligence, &c. Now if any one of differences should become marked and increase by intermarriage, on the principles of Natural Selection, or by the intentional internal of the Breeder, a new Race would be formed, and might be prop side by side with the old one. From the original staghound, still propagated its kind, twenty well-marked varieties might be reared, each of which would transmit its type. When we find an Egyptian plough closely resembling the plough still used in countries, we identify it as being of the same species; but not thereby disprove the fact that steam-ploughs and plough very various forms have been constructed side by side with form, all the new forms being modifications of the original type.

The answer to the question, Why, if species are variable they not varied during four thousand years? is thus ext simple. "Species," as a term designating a group of relat

not variable; but the parts related are variable; and when "Species" designates particular animals, we affirm that those animals which have been produced under similar conditions continue the type which has thence resulted, but those animals which have been produced under dissimilar conditions present corresponding variations from this type. There is one source of confusion which I shall more explicitly illustrate when treating of the relation between the Organism and its Medium; but as this would lead us too far from the course of our argument just now, I will merely say that by "conditions" we are not to understand geographical or climatal influences simply, or even mainly; but the whole group of conditions, external and internal, physical, organic, and social, which determine the result.

IV.

Passing now to the second question: Why are domesticated animals, if suffered to run wild, always found returning to the original type? This, which has been urged as a fatal objection against the hypothesis of Evolution, is, correctly interpreted, a necessary deduction from that hypothesis. I do not pause to discuss the validity of the statement itself, though Dr. Hooker and Mr. Darwin have pointed out the extremely imperfect evidence on which it is founded. I accept the argument as if there were no exaggeration in its data, and as if a domesticated animal suffered to run wild inevitably returned to the wild type; although, in the vast majority of cases, the animal would really perish, and instead of returning to the wild type would be supplanted by wild rivals, better suited to the medium. The argument fails even when its data are granted. Indeed, the very language of the objectors contains the terms of the answer. "Les variétés de plantes obtenues par le semis et les variétés d'animaux domestiques, loin d'être invoquées en faveur de la variabilité des espèces, sont, à mon sens, un puissant argument à l'appui de leur fixité." So speaks M. Chevreul; and we listen to such a master with attention. What is the argument? "Autrement," he says, "comment concevoir les difficultés que nous éprouvons à maintenir des modifications produites par la culture et le climat, ou par la domestication, lorsque nous les jugeons propre à satisfaire nos besoins ou nos jouissances? Dès que ces êtres modifiés cessent de se trouver dans les sphères des causes de modifications, celles-ci tendent à s'effacer."1 What more could Mr. Darwin desire than the admission that modifications, produced by the action of certain causes, disappear with the cessation of that action? If a plant or an animal changes under changed conditions, why should we conclude that on restoring it to the old conditions it will not again change in obedience to the same law? We have removed it from the sphere of later modifying causes, and

replaced it in the sphere of causes to which it was previously adapted by old modifications. Do you wonder that a steel spring rebounds when the pressure on it is removed? Do you wonder that it resists the pressure, and with every slackening of the pressure " tends " to return to the straight line from which you bend it? Why does it resist and rebound ? Because of a certain arrangement of its molecules. If you somewhat alter that arrangement, this alteration will give a permanent bend to the steel, and then there will be no force needed, no return to the straight line, except by the application of force. It is the same with plants and animals. A given type is the adaptation of structure to external conditions; the parts are so arranged that the organism can exist and continue its functions in this particular medium, and like the straight steel wire it can, within certain limits, be bent on the application of changed conditions. Yet, inasmuch as its arrangement of parts is one which was best adapted to the straight condition, it continually resists the forces which bend it, continually tends to recur to that state which is best adapted to its structure, and, consequently, no sooner are the ill-suited forces lessened or removed than we see a return to the original state. Every biologist knows that there is regressive, no less than progressive, metamorphosis; that an organ diminishes from disuse as it increases from use. It should also be remembered that in the modifications impressed on plants and animals under domestication, there is, for the most part, a change which serves our fancy and convenience rather than the advantage of the organism in its struggle for existence; and such changes difficult to be induced are naturally difficult to be maintained, so that they readily disappear when the modifying influences are removed. But those changes, which although perhaps brought about to suit our convenience, do likewise give the organism some advantage by its adaptation to the external conditions, will not readily alter when the organism is left to run wild; if it then alter, the alteration will be owing to external influences, not owing to internal tendencies regaining their old direction. Consider one example. Von Baer1 tells us that the zoologists of the sixteenth century expressly and unanimously declare that the guinea-pig was unknown in Europe before the discovery of America, yet that now the guinea-pig, as we know it, is only found in Europe. Our species is always variegated—black, brown, and white are its colours. The American is like it in size and form, but is always of a grey-brown colour. This, however, is a trifling difference. When we extend the comparison we find a wider gulf. The American loves damp places; the European perishes in them. The American supports a cold which destroys the European. The American brings forth young

only once a year; the European thrice. The changes in the osseous structure are not unimportant, but the change to which I would most call attention is that which, according to zoological dogmas, would constitute a generic difference—the American and European guinea-pigs will not couple together! Such have been the modifications consequent on three centuries of domestication. If we suppose our guinea-pig, which is well adapted to its medium, suddenly replaced in the medium of its ancestors, it would be so ill-adapted to that medium that it would inevitably perish; and if by a certain pliability of organisation it could readjust itself to the new condition, that readjustment would be through an approximation to the ancestral structure. Nothing can be more arbitrary than to assume modifying influences or external conditions in the case of domestication, and to deny a similar influence in the case of organisms removed to another sphere. " No one," says Mr. Darwin, "would expect that our improved pigs, if forced during several generations to travel about and root in the ground for their own subsistence, would transmit, as truly as they now do, their tendency to fatten, and their short muzzles and legs. Dray-horses assuredly would not long transmit their great size and massive limbs if compelled to live on a cold, damp mountainous region; we have, indeed, evidence of such deterioration in the horses which have run wild on the Falkland Islands. European dogs in India often fail to transmit their true character. Our sheep in tropical countries lose their wool in a few generations."

The tendency to recur to the ancestral form, a tendency noticeable even under domestication, is a fact of profound significance, but it is a simple consequence of biological laws, and is invoked by the advocates of Evolution not less than by the advocates on the other side. It has, therefore, no peculiar significance in the case now under examination. All we have to deal with here is the influence of external conditions in modifying an organism; and by a curious confusion of ideas, it is this very influence which is invoked to disprove an hypothesis founded on the possibilities of change under changing conditions.

V.

Respecting the third objection we have selected for discussion, namely, Why new species are not constantly appearing before our eyes, or why there is no evidence of such appearances in the existence of intermediate forms? it cannot conveniently be treated until we have treated of the way in which new forms arise; and for this discussion it will be necessary to expound certain biological principles. This I shall attempt in the second part. Meanwhile, if there were greater force in the objection than can be assigned to it, we should only regard it as a difficulty to be explained by future research, or to be interpreted on another form of the Development Hypothesis,

not as a refutation of that hypothesis. Indeed, we must protest against the frequent assumption that Darwinism is disproved because it fails to account for all the phenomena: if it interprets truly some of the phenomena, it is valuable as a colligation of facts; if it interpreted all of them it would cease to be an hypothesis. Observe, moreover, that writers who are most contemptuous against this hypothesis because it fails—or they think so—to explain some phenomena, urge us to accept the hypothesis of creative fiats, on Divine Ideas, which absolutely explain none. They reject an attempt to trace some of the intermediate steps by following the actual processes of evolution as far as these are known to us, and prefer relying on a vague phrase, which is only a restatement of the fact to be explained, and which suggests a process altogether inconceivable by the human mind.

At any rate, we have reached one result: Animals are variable The extent to which this variability may be carried under any gives Type is fairly a question; but we should remember that a Type is not a thing, but a relation: it is the arrangement of the parts which remains constant under a diversity in the size, shape, and number of those parts. The Vertebrate Type embraces all those animals which have an internal skeleton, and a neural axis above the hæmal axis. What amazing diversities it includes! Let us glance at its extreme limits—man and fish. Though both breathe air, one lives in the air and dies in the water; the other lives in the water and dies in the air. One breathes by lungs, the air entering through the nose a well as through the mouth; the other breathes by gills, the air entering only through the mouth. One is vocal, the other silent; on has limbs, the other none. One has four hearts, with double circulation and red blood corpuscles; the other two hearts, and single circulation (the Amphioxus has properly no heart, nor any red blood) Not only is the Vertebrate Type a purely ideal construction, representing the affinities of a large group, but it is varied in subordinate groups—mammals, marsupials, birds, reptiles, fishes; each group again having groups subordinated to it, and so on till we come to the group of Varieties, from which there are minor divergences not considered worthy of classification. "There are crustaceans," as Mr Darwin notices, "at the opposite ends of the series which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can recognised as unequivocally belonging to this, and to no other class of the Articulata." How fluctuating therefore must be the crustacean Type!

To return to our point: something is gained when the discussion of variability is disengaged from the misleading conception of Type and Species, and is reduced to the question of how far individually

forms can vary, and how far the accumulation of slight variations through successive generations may originate specific and generie distinctions. Naturalists, unable to deny the obvious fact of variation, have evaded the conclusion to which it points by boldly asserting that the variation is always confined to unimportant characters. It is not true, and Mr. Darwin has abundantly shown that it is not true. All organs vary. When naturalists assert—and no argument is more frequently used—that by Selection we have acted solely on the exterior, without in any respect altering the internal and essential parts ("sans en changer en rien la constitution essentielle et profonde"),1 the assertion is in one sense true, in another false; and the sense in which it is true does not oppose the Evolution hypothesis, whereas the sense in which it is false is an argument that upsets the hypothesis of fixed species. Thus it is true that the modification, which can be impressed on an individual, or on a succession of individuals, during a brief period are necessarily slight (and at first usually external), the laws of Adaptation rendering them so; but Mr. Darwin has nowhere intimated that the case was otherwise; indeed, his constant iteration of the principle that variations are slowly accumulated, ought to have prevented his adversaries from overlooking it—ought to have convinced them that the objection was beside the question. But while there is a sense in which it is true to say that the modifications are always slight, it is absurd to pretend that when these cumulate into striking alterations in the skeleton, alterations in the mode of alimentation, alteration in the modes of reproduction, alteration in the habits and instincts—of which there is overwhelming evidence—these are not essential alterations such as establish distinctions meriting the name of specific, nay also of generic. In his last work Mr. Darwin gives pictures of different breeds of pigeon, and, above all, of their skulls, which every naturalist would class as belonging to different species and genera, if he were unacquainted with their origin; but, being acquainted with their origin, he regards these diversities as proofs of "organic flexibility," and the trifling variations which species may assume. "If we could collect," says Mr, Darwin, "all the pigeons which have ever lived from before the time of the Romans to the present day, we should be able to group them in several lines, diverging from the parent rock pigeon. Each line would consist of almost insensible steps, occasionally broken by some slightly greater variation or sport, and each would culminate in one of our present highly modified forms." No less than one hundred and fifty distinct breeds have descended from one original stock, and these, if found in a state of nature, would have been grouped in at least five genera.

forth, we admit the fact that specific forms are persistent, but deny that this fact has the slightest value as evidence against the evolution of new specific forms through modification; and affirm that embryology furnishes the plainest testimony that such evolutions do take place. Species, except as a subjective classification of resemblance has no existence. Only individuals with variable resemblance exist; and as these individuals propagate, the propagation is necessarily a reproduction of the parent type. But while the law reproduction secures a continuance of the species, it also secures continuance of any variations from the parent form which may have been produced by incident forces sufficiently prolonged; and the variations may form the starting-points of divergence, from which time a new species will result.

Although this is a process by which all organic diversities may have been evolved, we are not obliged to accept it as more than an explanation of the way in which many of them have been evolved Natural Selection, although a true cause, is, I think, only one of the causes of diversity. There are many points which it leaves obscure and Mr. Darwin, with that noble calmness which distinguishes him admits the numerous difficulties. Whether these will hereafter be cleared away by an improvement in the Geological Record, now confessedly imperfect, or by more exhaustive exploration of distant countries, none can say; but, to my mind, the probability is that you shall have to seek our explanation by enlarging the hypothesis Natural Selection, subordinating it to the laws of Organic Combination. It does not seem to me, at present, warrantable to assured Descent as the sole principle of morphological uniformities; there are other grounds of resemblance beyond those of blood-relationship; and these have apparently been overlooked; yet a brief consideration we disclose that similarity in the laws and conditions of Organic Combination must produce similarity in organisms, independently relationship, just as similarity in the laws and conditions of inorganic combination will produce identity in chemical species. We do not suppose the carbonates and phosphates found in various parts of the globe—we do not suppose that the families of alkaloids and salts have any nearer kinship than that which consists in the similarity of the elements and the conditions of their combination. Hence, in organisms, as in salts, morphological identity may be due to a community of causal connection, rather than community of descent. Mr. Darwin justly holds it to be "incredible that individuals identically the same should have been produced through natural selection from parents specifically distinct," but he will not deny that identical forms may issue from parents genetically distinct, when these parent forms and the conditions of production are identical. To deny this would be to deny the law of causation. And that which is true of identical forms

under identical conditions is true of similar forms under similar conditions. When History and Ethnology reveal a striking uniformity in the progression of social phases, we do not thence conclude that the nations are directly related, or that the social forms have a common parentage, but that the social phases are alike because they have common causes. When chemists point out the uniformity of type which exists in compounds so diverse in many of their properties as water and sulphuretted or selenetted hydrogen, and declare phosphoretted hydrogen to be the congener of ammonia, they do not mean that the one is descended from the other, or that any closer link connects them than that of resemblance in their elements.

In the case of vegetal and animal organisms, we observe such a community of elementary substance as of itself to imply a community in their laws of combination, and under similar conditions the resulting forms must be similar. With this community of elementary substance, there are also diversities of substance and of conditions; corresponding with these diversities, there must be differences of form. Thus, although observation reveals that the bond of kinship does really unite many widely divergent forms, and the principle of Descent with Natural Selection will account for many of the resemblances and differences, there is at present no warrant for assuming that all resemblances and differences are due to this one cause, but, on the contrary, we are justified in assuming a deeper principle, which may be thus formulated: All the complex organisms are evolved from organisms less complex, as these were evolved from simpler forms; the link which unites all organisms is not always the common bond of heritage, but the uniformity of organic laws acting under uniform conditions.

It is therefore consistent with the hypothesis of Evolution to admit a variety of origins or starting-points, though not consistent to admit the sudden appearance of complex Types, such as is implied in the hypothesis of specific creations. I must reserve, however, for the second part of this essay, the grounds on which such a position may be defended.

PART II.

EVERY one who has pursued embryological researches, and in a lesser degree, every one who has merely read about them, must have been impressed by this marvel of marvels: an exceedingly minute portion of living matter, so simple in structure that a line will define it, passes by successive modifications into an organism so complex that a treatise is needed to describe it; not only does the simple cell, in which the ovum or the spermatozoon originates, pass into a complex organism, reproducing the forms and features of the parents (their longevity, their diseases, their mental dispositions, nay their very tricks and habits), but it may reproduce the form and features, the dispositions and diseases, of a grandfather or great-grandfather which had lain dormant in the father or mother. Consider for an instant what this implies. A microscopic cell of albuminous compounds, wholly without trace of organs, not appreciably distinguishable from millions of other cells, does nevertheless contain within it the "possibilities" of an organism so complex and so special as that of a Newton or a Napoleon. If ever there was a case when the famous Aristotelian notion of a "potential existence" seemed justified, assuredly it is this. And although we can only by a fallacy maintain the oak to be contained in the acorn, or the animal contained in the ovum, the fallacy is so natural, and indeed so difficult of escape, that there is no ground for surprise when physiologists, on first learning something of development, are found maintaining that the perfect organism existed already in the ovum, having all its lineaments in miniature, and only growing into visible dimensions through the successive stages of evolution.1 The preformation of the organism seemed an inevitable deduction from the opinions once universal. It led to many strange, and some absurd conclusions; among them, to the assertion that the original germ of every species contained within it all the countless individuals which in process of time might issue from it; and this in no metaphysical "potential" guise, but as actual boxed-up existences (emboités); so that Adam and Eve were in the most literal sense progenitors of the whole human race, and contained their progeny already shaped within them, awaiting the great accoucheur. Time.

This was the celebrated "emboîtement" theory. In spite of obvious objections it gained scientific acceptance, because physiologists could not bring themselves to believe that so marvellous a structure as

that of a human organism arose by a series of successive modifications, or because they could not comprehend how it was built up, part by part, into forms so closely resembling the parent-forms. That many and plausible reasons pleaded in favour of this opinion is evident in the fact that illustrious men like Haller, Bonnet, Vallisneri, Swammerdamm, Réaumur, and Cuvier, were its advocates; and if there is not a single physiologist of our day who accepts it, or who finds any peculiar difficulty in following the demonstrations of embryologists, how from the common starting-point of a selfmultiplying epithelial cell parts so diverse as hairs, nails, hoofs, scales, feathers, crystalline lens, and secreting glands may be evolved, or how from the homogeneous germinal membrane the complex organism will arise, there are very few among the scorners of the dead hypothesis who seem capable of generalising the principles which have destroyed it, or can conceive that the laws of Evolution apply as rigorously to the animal and vegetable kingdoms as to the individual organisms. The illustrious names of those who advocated the preformation hypothesis may serve to check our servile submission to the authorities so loudly proclaimed as advocates of the fixity of species. The more because the two doctrines have a common parentage. The one falls with the other; and no array of authorities can arrest the fall. That the manifold differentiations noticeable in a complex organism should have been evolved from a membrane wholly destitute of differences is a marvel, but a marvel which Science has made intelligible. Yet the majority of those to whom this has been made intelligible still find an impossibility in admitting that the manifold forms of plant and animal were successively evolved from equally simple origins. They relinquish the hypothesis of preformation in the one case, and cling to it in the other. Evolution, demonstrable in the individual history, seems preposterous in the history of the class. And thus is presented the instructive spectacle of philosophers laughing at the absurdities of "preformation," and yet exerting all their logic and rhetoric in defence of "creative fiats"—which is simply the preformation hypothesis writ large.

It would not be difficult to show that the doctrine of Epigenesis, with which Wolff for ever displaced the doctrine of Preformation, leads by an inevitable logic to the doctrine of universal Evolution; and that we can no more understand the appearance of a new organism which is not the modification of some already existing organism, than we can understand the sudden appearance of a new organ which is not the modification of some existing structure. In the one case as in the other we may disguise the process under such terms as creative fiat and preformation; but these terms are no explanations; they re-state the results, they do not describe the process; whereas Epigenesis describes the process as it passes under our eyes.

If any reader of these pages who, from theological or zoological suspicion of the Development Hypothesis, clings to the hypothesis of a creative Plan which once for all arranged the organic world in Types that could not change, will ask what rational interpretation can be given to the succession of phases each embryo is forced to pass through, it may help to give him pause. He will observe that none of these phases have any adaptation to the future state of the animal, but are in positive contradiction to it, or are simply purposeless; many of them have no adaptation even to its embryonic state; whereas all show stamped on them the unmistakeable characters of ancestral adaptations and the progressions of Organic Evolution. What does the fact imply? There is not a single known example of an organism which is not developed out of simpler forms. Before it can attain the complex structure which distinguishes it, there must be an evolution of forms which distinguish the structures of organisms lower in the series. On the hypothesis of a Plan which pre-arranged the organic world, nothing could be more unworthy of a supreme intelligence than this inability to construct an organism at once without previously making several tentative efforts, undoing to-day what was so carefully done yesterday, and repeating for centuries the same tentatives and the same corrections in the same succession. Do not let us blink this consideration. There is a traditional phrase much in vogue among the anthropomorphists, which arose naturally enough from the tendency to take human methods as an explanation of the divine—a phrase which becomes a sort of argument— "The Great Architect" (the emphasis of capitals seeming indispensable). But if we are to admit the human point of view, a glance at the facts of embryology must produce very uncomfortable reflections. For what should we say to an architect who was unable, or being able was obstinately unwilling, to erect a palace except by first using his materials in the shape of a hut, then pulling it down and rebuilding them as a cottage, then adding storey to storey and room to room, not with any reference to the ultimate purposes of the palace, but wholly with reference to the way in which houses were constructed in ancient times? What should we say to the architect who could not form a museum out of bricks and mortar, but was forced to begin as if going to build a mansion; and after proceeding some way in this direction, altered his plan into a palace, and that again into a museum? Would there be a chorus of applause from the Institute of Architects, and "favourable notices in the newspapers" of this profound wisdom? Yet this is the sort of succession on which organisms are constructed. The fact has long been familiar; how has it been reconciled with Infinite Wisdom? Let the following passage answer for a thousand:— "The embryo is nothing like the miniature of the adult. For a long while the body in its entirety and its details presents the strangest of spectacles. Day by day and hour by hour the aspect of the scene

changes, and this instability is exhibited by the most essential parts no less than by the accessory parts. One would say that Nature feels her way, and only reaches the goal after many times missing the path—on dirait que la nature tâtonne et ne conduit son œuvre à bon fin qu'après s'être souvent trompée."1 Writers have no compunction in speaking of Nature feeling her way and blundering; but if in lieu of Nature, which may mean anything, the Great Architect be substituted, it is probable that the repugnance to using such language of evasion may cause men to revise their conceptions altogether; they dare not attribute ignorance and incompetence to the Creator.

Obviously the architectural hypothesis is incompetent to explain the phenomena of organic development. Evolution is the universal process; not creation of a direct kind. Von Baer, who very properly corrected the exaggerations which had been put forth respecting the identity of the embryonic forms with adult forms lower in the scale, who showed that the mammalian embryo never was a bird, a reptile, or a fish, nevertheless emphasized the fact that the mammalian embryo passes through all the lower typical forms; so much so that, except by their size, it is impossible to distinguish the embryos of mammal, bird, lizard, or snake. "In my collection," he says, "there are two little embryos which I have omitted to label, so that now I am quite incompetent to say to what class they belong. They may be lizards, they may be small birds, or very young mammals; so complete is the similarity in the mode of formation of the head and trunk. The extremities have not yet made their appearance. But even if they existed in the earliest stage we should learn nothing from them, for the feet of lizards, mammals, and the wings of birds, all arise from the same common form." He sums up with his formula: "The special type is always evolved from a more general type."2

Such reminiscences of earlier forms are intelligible on the supposition that originally the later form was a modification of the earlier form, and that this modification is repeated; or on the supposition that there was a similarity in the organic conditions, which similarity ceased at the point where the new form emerged. But on no hypothesis of creative Plan are they intelligible. They are useless structures, failing even to subserve a temporary purpose. Sometimes, as Mr. Darwin remarks, a trace of the embryonic resemblance lasts till a late age: "Thus birds of the same genus, and of closely allied genera, often resemble each other in their first and second plumage; as we see in the spotted feathers in the thrush group. In the cat tribe most of the species are striped and spotted in lines; and stripes or spots can plainly be distinguished in the whelp of the lion and the puma. We occasionally, though rarely, see something

of this kind in plants.….. The points of structure in which the embryos of widely different animals of the same class resemble each other, often have no direct relation to their conditions of existence. We cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like courses of the arteries near the bronchial slits are related to similar conditions in the young mammal which is nourished in the womb of its mother, in the egg of a bird which is snatched in a nest, and in the spawn of a frog under water." It would be easy to multiply examples, but I will content myself with three. The tadpole of the Salamander has gills, and passes his existence in the water; but the Salamandra atra, which lives high up anong the mountains, brings forth its young full-formed. This animal never lives in the water. Yet if we open a gravid female, we find tadpoles inside her with exquisitely feathered gills, and (as I have witnessed) these tadpoles "when from the mother's womb untimely ripped," if placed in water, swim about like the tadpoles of water newts. Obviously this aquatic organisation has no reference to the future life of the animal, nor has it any adaptation to its embryonic condition; it has solely reference to ancestral adaptations, it repeats a phase in the development of its progenitors, Again, in the embryo of the naked Nudibranch, we always observe a shell, although the animal is without a shell, and there can be no purpose served by the shell in embryonic life.1 Finally, the human embryo has a tail, which is of course utterly purposeless, and which, although to be explained as a result of organic laws, is on the creative hypothesis only explained as an adherence to the general plan of structure—a specimen of pedantic trifling worthy of no intellect above be pongo's.2

Humanly appreciated, not only is it difficult to justify the successive stages of development, the incessant building up of structures immediately to be taken down, but also to explain why development was necessary at all. Why are not plants and animals formed at once, as Eve was mythically affirmed to be taken from Adam's rib, and Minerva from Jupiter's head? The theory of Descent answers this

(1) Curiously enough, while the Nudibranch, which is without a shell, possesses one during its embryonic life, there is another molluse, Neritina fluriatilis, which possessing shell in its subsequent life is without one during the early periods, and according to Claparede begins an independent existence capable of feeding itself before it acquires one. See his admirable memoir on the Neritina, in Müllers's Archiv., 1857.

(2) Has any advocate of the hypothesis that animals were created as we see them now, fully formed and wondrously adapted in all their parts to the conditions in which they live, ever considered the hind legs of the seal, which he may have watched in the Zoological Gardens? Here is an animal which habitually swims like a fish, and cannot use his hind limbs except as a rudder to propel him through the water; but instead of having a fish-like tail he has two legs flattened together, and nails on the toes—toes and nails being obvious superfluities. Now which is the more rational interpretation, that these limbs, in spite of their non-adaptation, were retained in rigid adherence to a Plan, or that the limbs were inherited from an ancestor who used them as legs, and that these legs have gradually become modified by the fish-like habits of the seal?

question very simply; the theory of Creation can only answer it by affirming that such was the ordained plan. But the theory of Descent not only gives the simpler and more intelligible answer to this question, it gives an answer to the further question which leaves the theory of Creation no loophole except a sophism—namely, why the formation of organisms is constantly being frustrated or perverted? And, further, it gives an explanation of the law noticed by Milne Edwards, that Nature is as economical in her means as she is prodigal in her variation of them: "On dirait qu'avant de recourir à des ressources nouvelles elle a voulu épuiser, en quelque sorte, chacun des procédés qu'elle avait mis en jeu."1 The applause bestowed on Nature for being so economical, is a curious transference to Nature of human necessities. Why, with a whole universe at her disposal, should Nature be economical? Why must she always be working in the same groove, and using but a few out of the many substances at her command? Economy is a virtue only in the poor. If Nature, in organic evolutions, is restricted to a very few substances, and a very few modes of combination, always creating new forms by modification of the old, and apparently incapable of creating an organism at once, this must imply an inherent necessity which is very unlike the free choice that can render economy a merit.

There may indeed be raised an objection to the Development Hypothesis on the ground that if the complex forms were all developed from the simpler forms, we ought to trace the identities through all their stages. If the fish developed into the reptile, the reptile into the bird, and the bird into the mammal (which I, for one, think very questionable), we ought to find, it is urged, evidence of this passage. And at one time it was asserted that the evidence existed; but this has been disproved, and on the disproof the opponents of Evolution take their stand. Although I cannot feel much confidence in the idea of such a passage from Type to Type, and although the passage, if ever it occurred, must have occurred at so remote a period as to leave no evidence more positive than inference, I cannot but think the teaching of Embryology far more favourable to it than to our opponents. Supposing, for the sake of argument, that the passage did take place, ought we to find the embryonic stages accurately reproducing the permanent forms of lower types? Von Baer thinks we ought; and lesser men may follow him without reproach. But it seems to me that he starts from an inadmissible assumption, namely, that the development must necessarily be in a straight line rather than in a multiplicity of divergent lines. "When we find the embryonic condition," he says, "differing from the adult, we ought to find a corresponding condition somewhere in the lower animals."2 Not necessarily. We know that the mental

development of a civilized man passes through the stages which the race passed through in the course of its long history, and the psychology of the child reproduces the psychology of the savage. But as his development takes place under conditions in many respects different, and as certain phases are hurried over, we do not expect to find a complete parallel. It is enough if we can trace general resemblances. Von Baer adds, "That certain correspondences should occur between the embryonic states of some animals and the adult states of others seems inevitable and of no significance(?). They could not fail, since the embryos lie within the animal sphere and the variations of which the animal body is capable are determined for each type by the internal connection and mutual reaction of its organs so that particular repetitions are inevitable." A profound remark, to which I shall hereafter have occasion to return, but its hearing on the present question is inconclusive. The fact that the embryonic stages of the higher animals resemble in general characters the permanent stages of the lower animals, and very closely resemble the embryonic stages of those animals, is all that the Development Hypothesis requires. Nor is its value lessened by the fact that many of the details and intermediate stages seem passed over in the development of the higher forms, for the recapitulation can only be of outlines not of details; since there are differences in the forms there must be differences in their histories.

In the preceding observations the object has simply been to show that the phenomena to be explained can be rationally conceived as resulting from gradual Evolution, whereas they cannot be rationally interpreted on any other hypothesis. And here it may be needful to say a word respecting Epigenesis.

The Preformation hypothesis, which regarded every organism is a simple educt and not the product of a germ, was called by its advocates an evolution hypothesis—meaning that the adult form was an outgrowth of the germ, the miniature magnified. Wolff, who replaced that conception by a truer one, called his, by contrast, Epigenesis, meaning that there was not simply out-growth but new growth. "The various parts," he says, "arise one after the other, to that always one is secreted from (excernirt), or deposited (deponirt) on the other; and then it is either a free and independent part, or is only fixed to that which gave it existence, or else is contained within it. So that every part is the effect of a pre-existing part, and in turn the cause of a succeeding part."1 The last sentence expresses the conception of Epigenesis which embryologists now adopt; and having said this, we may admit that Wolff, in combating the error of preformation, replacing it with the truer notion of gradual and successive formation, was occasionally open to the criticism made by Von Baer, that he missed the true sense of Evolution, since the new

parts are not added on to the old parts as new formations, but evolved from them as transformations. "The word Evolution, therefore, seems to me more descriptive of the process than Epigenesis. It is true that the organism is not preformed, but the course of its development is precisely the course which its parents formerly passed through. Thus it is the Invisible—the course of development—which is predetermined."1 When the word Epigenesis is used, therefore, the reader will understand it to signify that necessary succession which determines the existence of new forms. Just as the formation of chalk is not the indifferent product of any combination of its elements, carbon, oxygen, and calcium, but is the product of only one series of combinations, an evolution through necessary successions, the carbon uniting with oxygen to form carbonic acid, and this combining with the oxide of calcium to form chalk, so likewise the formation of a muscle, a bone, a limb, or a joint has its successive stages, each of which is necessary, none of which can be transposed. The formation of bone is peculiarly instructive, because the large proportion of inorganic matter in its substance, and seemingly deposited in the organic tissue, would lead one to suppose that it was almost an accidental formation, which might take place anywhere; yet although what is called connective tissue will ossify under certain conditions, true bone is the product of a very peculiar modification, which needs to be preceded by cartilage. That the formation of bone has its special history may be seen in the fact that it is the last to appear in the animal series, many highly-organized fishes being without it, and all the other systems appearing before it in the development of the embryo. Thus although the mother's blood furnishes all the requisite material, the fœtus is incapable of assimilating this material and of forming bone, until its own development has reached a certain stage. Moreover, when ossification does begin, it begins in the skull; and the only approach to an internal skeleton in the Invertebrates is the so-called skull of the Cephalopoda. Not only is bone a late development, but cartilage is also; and although it is an error to maintain that the Invertebrates are wholly destitute of cartilage, its occasional presence having been fully proved by Claparède and Gegenbaur, the rarity of its presence is very significant. The animals which can form shells of chalk and chitine, are yet incapable of forming even an approach to bone.

Epigenesis depends on the laws of succession, which may be likened to the laws of crystallisation, if we bear in mind the essential differences between a crystal and an organism, the latter retaining its individuality through an incessant molecular change, the former only by the exclusion of all change. When a crystalline solution takes shape, it will always take a definite shape, which represents what may he called the direction of its forces, the polarity of its con-

stituent molecules. In like manner, when an organic plasma takes shape—crystallises, so to speak—it always assumes a specific shape dependent on the polarity of its molecules. Crystallographers have determined the several forms possible to crystals; histologists have recorded the several forms of anatomical Elements, Tissues, and Organs. Owing to the greater variety in elementary composition, there is in organic substance a more various polar distribution than in crystals; nevertheless there are sharply defined limits never overstepped, and these constitute what may be called the specific forms of Elements, Tissues, Organs, Organisms. An epithelial cell, for example, may be ciliated or columnar, a muscle-fibre striated or non-striated, a nerve-fibre naked or enveloped in a sheath, but the kind is always sharply defined. A nerve-centre may be a single ganglion with two nerves issuing from it, or a fused mass of ganglia with many outgoing nerves. An intestinal tube may be an uniform canal, or a canal differentiated into several unlike compartments, with several unlike glandular appendages. A spinal column may be an uniform solid axis, or a highly diversified segmented axis. A limb may be an arm, or a leg, a wing, or a paddle. In every case the anatomist recognises a specific type. He assigns the uniformities to the uniformity of the substance thus variously shaped, under a history which has been similar; the diversities he assigns to the various conditions under which the processes of growth have been determined, He never expects a muscular tissue to develop into a skeleton, a nervous tissue into a gland, an osseous tissue into a sensory organ. He never expects a tail to become a hand or a foot, though he sees, in monkeys and marsupials serving the offices of prehension and Locomotion. He never expects to find fingers growing anywhere except from metacarpal bones, or an arm developed from a skull. The well known generalisation of Geoffroy St. Hilaire that an organ is more easily annihilated than transposed, points to the fundamental law of Epigenesis. In the same direction point all the facts of growth. Out of a formless germinal membrane we see an immense variety of forms evolved; and out of a common nutritive fluid this variety of organs is sustained, repaired, replaced; and this not differently, not casually, but according to rigorous laws of succession, that which precedes determining that which succeeds as inevitably as youth precedes maturity, and maturity decay. The nourishment of various organs from a common fluid, each selecting from that fluid only those molecules that are like itself, rejecting all the rest, is very similar to the formation of various crystals in a solution of different salts, each salt separating from the solution only those molecules that are like itself. Reil long ago called attention to this analogy. He observed that if in a solution of nitre and sulphate of soda a crystal of nitre be dropped, all the dissolved nitre crystallises, the sulphate remaining in solution; whereas on reversing the experi-

ment, a crystal of sulphate of soda is found to crystallise all the dissolved sulphate, leaving the nitre undisturbed. In like manner muscle selects from the blood its own materials which are there in solution, rejecting those which the nerve will select.

Nay, so definite is the course of growth, that when a limb or part of a limb is cut off from a crab or a salamander, a new limb or new part is reproduced in the old spot, exactly like the one removed. Bonnet startled the world by the announcement that the Nais, a worm common in ponds, spontaneously divided itself into two worms; and that when he cut it into several pieces, each piece reproduced head and tail and grew into a perfect worm. This had been accepted by all naturalists without demur, until Dr. Williams, in his "Report on British Annelida, 1851," declared it to be a fable. In 1858, under the impulse of Dr. Williams's very emphatic denial, I repeated experiments similar to those of Bonnet, with similar results. I cut two worms in half, and threw away the head-bearing segments, placing the others in two separate vessels, with nothing but water and a little mud, which was first carefully inspected to see that no worm lay concealed therein. In a few days the heads were completely reformed, and I had the pleasure of watching them during their reconstruction. When the worms were quite perfect, I again cut away their heads, and again saw these reformed. This was repeated, till I had seen four heads reproduced; after which the worms succumbed.

The question naturally arises, Why does the nutritive fluid furnish only material which is formed into a part like the old one, instead of reproducing another part, or one having a somewhat different structure? The answer to this question is the key to the chief problem of organic life. That a limb in situ should replace its molecular waste by molecules derived from the blood, seems intelligible enough (because we are familiar with it), and may he likened to the formation of crystals in a solution; but how is it that the limb which is not in existence can assimilate materials from the blood? how is it that the blood, which elsewhere in the organism will form other parts, here will only form this particular part? There is, probably, no one who has turned his attention to these subjects who has not paused to consider this mystery. The most accredited answer at present before the world is one so metaphysiological that I should pass it by, were it not intimately allied with that conception of Species, which it is the object of these pages to root out. It is this:

The organism is determined by its Type, or, as the Germans say, its Idea. All its parts take shape according to this ruling plan; consequently, when any part is removed, it is reproduced according to the Idea of the whole of which it forms a part. Milne Edwards, in a very interesting and suggestive work, concludes his survey of organic phenomena in these words: "Dans l'organisme tout semble calculé en vue d'un résultat déterminé, et l'harmonie des parties ne résulte pas de

l'influence qu'elles peuvent exercer les unes sur les autres, mais de leur co-ordination sous l'empire d'une puissance commune, d'un plan préconçu, d'une force pré-existante."1 This is eminently metaphysiological. It refuses to acknowledge the operation of immanent properties, refuses to admit that the harmony of a complex structure results from the mutual relations of its parts, and seeks outside the organism for some mysterious force, some plan, not otherwise specified, which regulates and shapes the parts. Von Baer, in his great work, has a section entitled, "The nature of the animal determines its development;" and he thus explains himself: "Although every stage in development is only made possible by the pre-existing condition which is another mode of expressing Epigenesis], nevertheless the entire development is ruled and guided by the Nature of the animal which is about to be (von der gesammten Wesenheit des Thieres welches werden soll), and it is not the momentary condition which alone and absolutely determines the future, but more general and higher relations."2 One must always be slow in rejecting the thoughts of a master, and feel sure that one sees the source of the error before regarding it as an error; in the present case I think the positive biologist will be at no loss to assign Von Baer's error to its metaphysical origin. Without pausing here to accumulate examples both of anomalies and slighter deviations which are demonstrably due to the "momentary conditions" that preceded them, let us simply note the logical inconsistency of a position which, while assuming that every separate stage in development is the necessary sequence of its predecessor, declares the whole of the stages independent of such relations! Such a position is indeed reconcileable on the assumption that animal forms are moulded "like clay in the hands of the potter." But this is a theological dogma, which leads (as we saw, pp. 613 and 614) to very preposterous and impious conclusions; and whether it leads to these conclusions or others, positive Biology declines theological explanations altogether. Von Baer, although he held the doctrine of Epigenesis, coupled it, as many others have done, with metaphysical doctrines to which it is radically opposed. He believed in Types as realities; he was therefore consistent in saying, "It is not the Matter and its arrangements which determine the product, but the nature of the parent form—the Idea, according to the new school.". How are we to understand this Idea? If it mean an independent Entity, an agency external to the organism, we refuse to acknowledge its existence. If it mean only an à posteriori abstraction, expressing the totality of the momenta, then indeed we acknowledge that it determines the animal form; but this is only an abbreviated way of expressing the law of Evolution, by which each stage determines its successor. The Type does not dominate the momenta, it emerges from them; the animal organism

is not cast in a mould, but the imaginary mould is the form which the polarities of the organic substance assume. It would seem very absurd to suppose that crystals assumed their definite shapes (when the liquid which held their molecules in solution is evaperated) under the determining impulse of phantom-crystals, or Ideas; yet it has not been thought absurd to assume phantom forms of organisms.

The conception of Type as a determining influence arises from that fallacy of taking a resultant for a principle, which has played so conspicuous a part in the history of philosophy. Like many others of its class it exhibits an interesting evolution from the crude metaphysical to the subtle metaphysical point of view, which at last insensibly blends into the positive point of view. At first the Type or Idea was regarded as an objective reality, external to the organism it was supposed to rule. Then this notion was replaced by an approach to the more rational interpretation, the Idea was made an internal not an external force, and was incorporated with the material elements of the organism, which were said to "endeavour" to arrange themselves according to the Type. Thus Treviranus declares that the seed "dreams of the future flower;" and "Henle, when he affirms that hair and nails grow in virtue of the Idea, is forced to add that the parts endeavour to arrange themselves according to this Idea."1 Even Lotze, who has argued so victoriously against the vitalists, and has made it clear that an organism is a mechanism, cannot relinquish this conception of legislative Ideas, though he significantly adds, "these have no power in themselves, but only in as far as they are grounded in mechanical conditions." Why then superfluously add them to the conditions? If every part of a watch, in virtue of the properties inherent in its substance, and of the mutual reactions of these and other parts, has a mechanical value, and if the sum of all these parts is the time-indicating mechanism, do we add to our knowledge of the watch, and our means of repairing or improving it, by assuming that the parts have over and above their physical properties the metaphysical "tendency" or "desire" to arrange themselves into this specific form? When we see that an organism is constructed of various parts, each of which has its own properties inalienable from its structure, and its uses dependent on its relation to other parts, do we gain any larger insight by crediting these parts with desires or "dreams" of a future result which their union will effect? That which is true in this conception of legislative Ideas is that when the parts come together there is mutual reaction, and the resultant of the whole is something very unlike the mere addition of the items, just as water is very unlike oxygen or hydrogen; further, the consensus of the whole impresses a peculiar direction on the development of the parts, and the law of Epigenesis necessitates a serial development, which may easily be interpreted as due to a preordained plan.

In a word, this conception of Type only adds a new name to the old difficulty, adding mist to darkness. The law of Epigenesis, which is simply the expression of the material process determined by the polarity of molecules, explains as much of the phenomena as is explicable. A lost limb is replaced by the very processes and through the same progressive stages as those which originally produced it. We have a demonstration of its not being reformed according to any Idea or Type which exists apart from the immanent properties of the organic molecules, in the fact that it is not reformed at once, but by gradual evolution; the mass of cells at the stump are cells of embryonic character, cells such as those which originally "crystalised" into muscles, nerves, vessels, and integument, and each cell passes through all its ordinary stages of development. It is to be remembered that so intimately dependent is the resultant on the determining momenta that any external influence which disturbs the normal course of development will either produce an anomaly, or frustrate the formation of a new limb altogether. One of my ritons bit off the leg of his female;1 the leg which replaced it was much malformed, and curled over the back so as to be useless; was his according to the Idea? I cut it off, and examined it; all the ones were present, but the humerus was twisted, and of small size. In a few weeks a new leg was developed, and this leg was normal. If the Idea, as a ruling power, determined the growth of this third leg, what determined the second, which was malformed? Are we to suppose that in normal growth the Idea prevails, in abnormal the conditions? That it is the polarity of the molecules which at each moment determines the group those molecules will assume, is well seen in the experiment of Lavalle mentioned by Bronn.2 He showed hat if when an octohædral crystal is forming, an angle be cut away o as to produce an artificial surface, a similar surface is produced pontaneously on the corresponding angle, whereas all the other angles are sharply defined. "Valentin," says Mr.Darwin, "injured the caudal extremity of an embryo, and three days afterwards it produced rudiments of a double pelvis and of double hind limbs. Hunter and others have observed lizards with their tails reprodueed and doubled. When Bonnet divided longitudinally the foot of the salamander several additional digits were occasionally formed."3 Where is the evidence of the Idea in these cases?

(1) I had kept these tritons four years in the hope that they would breed; but in spite of their being subjected to great varieties of treatment, for months well supplied with food, and for months reduced almost to starvation, they never showed the slightest tendency to breed; another among the many illustrations of the readiness with which the generative system is affected even in very hardy and not very impressionable animals. CLAPAREDE observed the still more surprising fact that the Neritina fluriatilis (a river snail) not only will not lay eggs, but will not even feed in captivity. He attributes it to the stillness of the water in the aquarium, so unlike that of tho running streams in which the molluac lives. See Müller's Archiv., 1857.

(2) BRONN: Morphologische Studion über die, Gestaltung-Gesetze, 1858.

(3) DARWIN: On Domestication, ii. 340. In the Annales des Sciences, 1862, p. 358, M. MALM describes a fish in his collection the tail of which had been broken, and the bone which grow out at the injured spot had formed a second tail with terminal fln.

I repeat, the reproduction of lost limbs is due to a process which is in all essential respects the same as that which originally produced them; the genesis of one group of cells is the necessary condition for the genesis of its successor, nor can this order be transposed. But—and the point is very important—it is not every part that can be reproduced, nor is it every animal that has reproductive powers. The worm, or the mollusc, seems capable of reproducing every part; the crab will reproduce its claws, but not its head or tail; the insect will reproduce no part (indeed the amputation of its antennæ only is fatal), the salamander will reproduce its leg, the frog not. In human beings a muscle is said never to be reproduced; but this is not the case in the rare examples of supplementary fingers and toes, which have been known to grow again after amputation. The explanation of this difference in the reproductive powers of different animals is usually assigned to the degree in which their organisms retain the embryonic condition; and this explanation is made plausible by the fact that the animals which when adult have no power of replacing lost limbs, have the power when in the larval state. But although this may in some cases be the true explanation, there are many in which it fails, as will be acknowledged after a survey of the extremely various organisms at widely different parts of the animal series which possess the reproductive power. Even animals in the same class, and at the same stage of development, differ in this respect. I do not attach much importance to the fact that all my experiments on marine annelids failed to furnish evidence of their power of reproducing lost segments; because it is difficult to keep them under conditions similar to those in which they live. But it is significant that, among the hundreds which have passed under my observation, not one should have been found with a head-segment in the process of development, replacing one that had been destroyed; and this is all the more remarkable from the great tenacity of life which the mutilated segments manifest. Quatrefages had observed portions of a worm, after gangrene had destroyed its head and several segments, move about in the water and avoid the light!1

A final argument to show that the reproduction is not determined by any ruling Idea, but by the organic conditions and the necessary stages of evolution, is seen in the re-appearance of a tumour or cancer after it has been removed. We find the new tissue appear

(1) In the valuable memoir on the Anatomy and Physiology of the Nematoids, by Dr. CHARLTON BASTIAN, which appeared in the Philosophical Transactions for 1866, we read that even these lowly-organised worms have little power of repair. Speaking of the "paste eels" (Anguilulidœ) he says, "I may state as the result of many experiments? with these that the power they possess of repairing injuries seems very low. I hare cut off portions of the posterior extremity, and though I watched the animal for days after, could never recognise any attempt at repair." Perhaps, however, the season may have some influence; and Dr. WILLIAM'S denial respecting the Nais may be thus explained.

with all the characters of the normal tissue of the gland, then rapidly assume one by one the characters of the diseased tissue which had been removed; and the reason is, that the regeneration of the tissue is accompanied by the same abnormal conditions which formerly gave rise to the tumour: the directions of "crystallisation" are similar because the conditions are similar. In every case of growth or regrowth the momenta being the same, the result must be the same.

It seems a truism to insist that similarity in the results must be due to similarity in the conditions, yet it is one which many theorists disregard, and especially do we need to bear it in mind when arguing about Species. It is this law of causation which may occasionally be invoked against Mr. Darwin himself, who seems to me inclined to attribute resemblances to kinship which might more properly be attributed to a community of conditions. As I shall in future papers have several opportunities of discussing this point in detail, I will here only touch on the suggestive topic of the analogies observed not simply among animals at the extreme ends of the scale, but also between animals and plants where the idea of a direct kinship is out of the question.

My very imperfect zoological knowledge will not allow me to adduce a long array of instances, but such an array will assuredly occur to every well-stored mind, for I have been frequently impressed with the occurrence of analogies in the remotest quarters. It is enough to point to the many analogies of Function, more especially in the reproductive processes—to the existence of burrowers, waders, flyers, swimmers in various classes—to the existence of predatory mammals, predatory birds, predatory reptiles, predatory insects by the side of herbivorous congeners,—to the nest-building and incubating fishes; and in the matter of Structure the analogies are even more illustrative when we consider the widely-diffused spicula, setæ, spines, hooks, tentacles, beaks, feathery forms, nettling-organs, poison-sacs, luminous organs, &c., because these have the obvious impress of being due to a community of substance under similar conditions rather than to a community of kinship. The beak of the tadpole, the cephalopod, the male salmon, and the bird, are no doubt in many respects unlike; but there is a significant likeness among them, which constitutes a true analogy. I think there is such an analogy between the airbladder of fishes and the tracheal rudiment which is found in the gnatlarva (Corethra plumicornis).1 Very remarkable also is the resemblance

(1) This beautiful and transparent larva reminds one in many respects of the Pike as it poises itself in the water awaiting its prey. It is enabled to do so without the slightest exertion by the air-bladders which it possesses in the two kidney-shaped rudiments of tracheæ, and which in the gnat become developed into the respiratory apparatus. The resemblance to the air-bladder of fishes is not simply that it serves a similar purpose of sustaining the body in the water, it is in both cases a rudiment of the respiratory apparatus, which in the flsh never becomes developed. WRISMANN calls attention to an organ in the larvæ of certain insects (the Culicidœ) which

of the avicularium, or "bird's-head process," on the polyzoon known popularly as the Corkscrew Coralline (Bugula avicularia), which presents us in miniature with a vulture's head—two mandibles, one fixed, the other moved by muscles visible within the head. No one can watch this organ snapping incessantly without being reminded of a vulture, yet no one would suppose for a moment that the resemblance has anything to do with kinship.

Such cases are commonly robbed of their due significance by being dismissed as coincidences. But what determines the coincidence? If we assume, as we are justified in assuming, that the possible directions of Organic Combination and the resultant forms are limited, there must inevitably occur such coincident lines; and the hooks on a Climbing Plant will resemble the hooks on a Crustacean or the claws of a Bird, as one form in which under similar external forces the more solid but not massive portions of the integument tend to develop. I am too unacquainted with the anatomy of plants to say how the hooks so common among them arise; but from examination of the Blackberry, and comparison of its thorns with the hooks and spines of the Crustacea, I am led to infer that in each case the mode of development is identical—namely, the secretion of chitine from the cellular matrix of the integument.

Another mode of evading the real significance of such resemblances is to call them analogies, not homologies. There is an advantage in having two such terms, but we ought to be very clear as to their meaning and their point of separation. Analogy is used to designate similarity in Function with dissimilarity in Structure. The wing of an insect, the wing of a bird, and the wing of a bat, are called analogous, but not homologous, because their anatomical structure is different: they are not constructed out of corresponding elements. The foreleg of a mammal, the wing of a bird, or the paddle of a whale, are called homologous, because in spite of their diverse uses they are constructed out of corresponding anatomical parts. To the anatomist such distinctions are eminently serviceable. But they have led to some misconceptions, because they are connected with a profound misconception of the relation between Function and Organ, which we shall have to consider in our next article. Embryology teaches that the wing of the bird and the paddle of the whale are developed out of corresponding parts, and that these are not like the parts from which the wing of an insect or the flying-fish will be developed; nevertheless, the most cursory inspection reveals that the wing of a bird and the paddle of a whale are very unlike in structure no less than in function, and that their diversities in function correspond

has what ha calls a tracheal gill, and which has this striking analogy with the fish-gill that it separates the air from the water, and not, as a trachea, direct from the atmosphere. See his remarkable memoir Die nachembryonale Entwickelung des Muscidens, in Siebold und Kolliker's Zeitschrift, 1864, p. 223.

with their diversities in structure; whereas the wing of an insect, of a bird, and of a bat, are in fundamental characters very similar; and, corresponding with such similarities, there are similarities in function. In fact, both Analogy and Homology imply that amid a diversity of momenta, expressed in the variety of the results, there are certain momenta in common, which are expressed in the resemblance of the results. Thus, to take an extreme case, the suffocation which results from drowning and that which results from breathing an atmosphere deprived of its oxygen, are phenomena very different in many of their conditions; but that essential point which constitutes the analogy is the same in both, namely, the prevention of the exit of carbonic acid from the blood. Dr. Paris, in his "Pharmacologia," observes that an extensive list of animal substances have been discarded from the Materia Medica since it has been known that their properties are due to the same principle—i.e., gelatine, albumen, carbonate of lime, &c.; and again that every animal substance containing nitrogen yields ammonia. Such discoveries have banished earthworms, vipers skinned and deprived of their entrails, human skulls, dried blood, elk's hoof, urine of a child, &c. Thus amid a variety of substances producing similarity of effects there waa detected one element common to them all, and it was this which produced the effect similar in all. It is important to bear in mind that whenever an analogy occurs it is founded on a corresponding similarity in the momenta; and that the wings of an insect, a bird, and a bat, although variously formed, and therefore not homologous, are also similarly formed, and therefore analogous, the analogy both of structure and function being due to, and limited by, the amount of resemblance; for in truth, although we say that the flight of an insect, a bird, and a bat is "the same thing," this is very loose speech, and very misleading. The flight is different in each case, the weight to be moved, the rapidity with which it is moved, the precision of the movements, the endurance of the movements, all differ, all depend on differences in the mechanism; on the other hand, they agree in being methods by which the force of gravity is overcome, and the body propelled through the air by a rapidly-moving instrument; and for this the conditions require a light organ capable of pressing on a large surface, and moved by powerful muscles: whether it be formed of feathers or membrane the physical result will be similar.

I shall have hereafter to recall this point in discussing whether the analogies of organisms imply a community of kinship, or, as I maintain, simply a community in organic laws. Homologies may be thought more decisively to point to kinship, and very often they do so beyond a doubt; but we shall see how impossible it is to draw the line between homologies and analogies in many cases; and shall be compelled to recur to the more general statement that similarity in the conditions necessarily produces similarity in the results. This view is, however, too

divergent from the one current among biologists to be accepted without some preliminary explanation, and I must ask the reader to suspend his judgment until the grounds have been laid open to him. He is requested not to suppose that by "conditions" are meant the external conditions only, to the exclusion of the organism: "the conditions" involve the whole of the causes converging to a given result.

When we see an annelid and a vertebrate resembling each other in some special point which is not common either to their classes or to any intermediate classes—as when we see the woodlouse (Oniscus)and the hedgehog defend themselves in the same strange way by rolling up into a ball—we cannot interpret this as a trace of distant kinship. When we see a breed of pigeons and a breed of canaries turning summersaults, and one of the Bear family (Ratel) given to the same singular habit, we can hardly suppose that this is in each case inherited from a common progenitor. When we see one savage race tipping arrows with iron, and another, ignorant of iron, using poison, there is a community of object effected by diversity of means, but the analogy does not necessarily imply any closer connection between the two races than the fact that men with similar faculties and similar wants find out similar methods of supplying their wants. Even those who admit that the human race is one family, and that the various peoples carried with them a common fund of knowledge when they separated from the parent stock, may still point to a variety of new inventions and new social developments which occurred quite independently of each other, yet are strikingly alike. Their resemblance will be due to resemblance in the conditions. The existence, for example, of a religious worship, or a social institution, in two nations widely separated both in time and space, and under great historical diversities, is no absolute proof that these two nations are from the same stock, and that the ideas have the same parentage. It may be so; it may be otherwise. It may be an analogy no more implying kinship than the fact of ants making slaves of other ants (and these the block ants!) implies a kinship with men. Given an organisation which in the two nations is alike, and a history which is in certain characteristics analogous, there must inevitably result religious and social institutions having a corresponding resemblance. I do not wish to imply that the researches of philologists and ethnologists are misdirected, or that their conclusions respecting the kinship of man kind are to be rejected; I only urge the consideration that perhaps too much stress is laid on community of blood, and not enough on community of conditions. And here, again, this phrase reminds me that until certain principles of Biology have been agreed upon, nothing but misconception of my position can be expected. In the next paper some attempt will be made to expound these principles and apply them.

FORTNIGHTLY

REVIEW.

JOHN MORLEY.

PART III.

THE simplest form of organic life is not—as commonly stated—a cell, but a microscopic lump of jelly-like substance, or protoplasm, which has been named sarcode by Dujardin, cytode by Haeckel, and germinal matter by Lionel Beale. This protoplasm, although entirely destitute of texture, and consequently destitute of organs, is nevertheless considered to be an Organism, because it manifests the cardinal phenomena of Life: Nutrition, Reproduction, and Contractility. As examples of this simplest organism we may cite Monads, Vibriones, Protamœbæ, and Polythalamia.1 Few things are more surprising than the vital activity of these organisms, which puzzle naturalists as to whether they should be called plants or animals. All microscopists are familiar with the spectacle of a formless lump of albuminous matter (a Rhizopod), putting forth a process of its body as a temporary arm or leg, or else slowly wrapping itself round a microscopic plant, or morsel of animal substance, thus making its whole body a mouth and a stomach; but these phenomena are as nothing to those described by Cienkowski,2 who narrates how one Monad fastens on to a plant and sucks the chlorophyl, first from one cell and then from another; while another Monad, unable to make a hole in the cell-wall, thrusts long processes of its body into the opening already made, and drags out the remains of the chlorophyl left there by its predecessor; while a third Monad leads a predatory life, falling upon other Monads who have filled themselves with food. Here, as he says, we stand on the threshold of that dark region where Animal Will begins; and yet there is here no trace of organisation.

Now let our glance pass on to the second stage—the Cell. Here we have the first recognised differentiation of structure, in the appearance of a nucleus, or attractive centre, amid the protoplasm. The nucleus is chemically different from the substance which surrounds it; and although perhaps exaggerated importance has been attributed to this nucleus, and mysterious powers have been ascribed to it, yet as an essential constituent of the Cell it commands attention. Indeed, according to the most recent investigations, the definition of a Cell is "a nucleus with surrounding protoplasm." The cell-wall, or delicate investing membrane—that which makes

the Cell a closed sac—is no longer to be regarded as a necessary constituent, but only as an accessory.1

The cell is an organism. It may lead an isolated life as plant or animal, or it may be united with others and lead a more or less corporate existence; but always, even as an element of a higher organism, it preserves its own individuality; it is born, is developed, and decays; it runs its own course, irrespective of associates. At first we see that the corporate union is very slight, merely the contact of one cell with another of its own kind, as in the filament of a Conferva. Rising higher, we see the cell united with others different from it: plants and animals appear, having structures composed of masses of various cells. Rising still higher, we see animal forms of which the web is woven out of myriads upon myriads of cells, with various cell-products, processes, fibres, tubes.

But we have only one half of the great problem of life, when we have the Organism; and it is to this half that the chief researches have been devoted, the other falling into neglect. What is that other? The Medium in which the Organism lives. Every individual object, organic or inorganic, is the sum of two factors:—first, the relation of its constituent molecules to each other; secondly, the relation of its substance to all surrounding objects. Its properties, as an object or an organism, are the resultant of its constituent molecules, and of its adaptation to external conditions. Organisms are the resultants of a peculiar group of forces, exhibiting a peculiar group of phenomena. Viewing these in the abstract, we may say that there are three regulative laws of life:—(1) The Lex Formationis—the so-called nisus formativus, or "organising force;" (2) the Lex Adaptationis, or adaptive tendency; (3) the Lex Hereditatis, or tendency to reproduce both the original form and its acquired modifications. We have always to consider the organising force in relation to all surrounding forces—a relation succinctly expressed in the word Adaptation. Just as water is water only under a certain relation of its constituent molecules to the temperature and atmospheric

(1) In the cell-theory established by SCHLEIDEN and SCHWANN, in 1838, and which has formed the basis of modern histology, the cell-wall was endowed with an importance which can no longer be upheld now that the existence of independent organisms, and of cells, without a trace of enveloping membrane has been abundantly observed. MAX SCHULTZE, in his famous essay, Ueber Muskelkörperehen und was man eiue celle zu nennen habe, which appeared in Reichert und Du Bois Reymond's Archiv, 1861,—BRUECKE, in his memoir Die Elementarorganismen, 1861,—and LIONEL BEALE, in his Structure of the Simple Tissues, 1861,—all about the same time began the reform in the cell-theory which has effected a decisive change in the classical teaching. LEYDIG claims, and with justice, to have furnished important data in this direction, Vom Bau des thierischen Körpers, 1864, I. p. 11. The student interested in this discussion should consult MAX SCHULTZE: Das Protoplasma der Rhizopoden und der Pflanzenzellen, 1863; HAECKEL: Die Radiolarien, 1862; and the controversial papers by REICHERT, in his Archiv (beginning with the Report of 1863), and MAX SCHULTZE, in his Archiv für mikros. Anat., with HENLE'S judgment in his Jahresberichte, and KÖLLIKER'S summing-up in the last edition of his Gewebelehre.

pressure—just as it passes into other forms (ice or steam) in adapting itself to other conditions; so, likewise, the Organism only preserves its individuality by synchronising its forces with the forces which environ it.

This relation of Organism and Medium, the most fundamental of biological data, has had a peculiar fortune: never wholly unrecognised, for it obtrudes itself incessantly in the facts of daily experience, it was very late in gaining recognition as a principle of supreme importance; and is even now often so imperfectly apprehended that one school of philosophers indignantly rejects the idea of the Organism and Medium being the two factors of which Life is the resultant. Not only is there a school of vitalists maintaining the doctrine of Life as an entity independent both of Organism and Medium, and using these as its instruments; but there is also a majority among the more scientific biologists, who betray by their arguments that they fail to keep steadily before them the fundamental nature of the relation. Something of this is doubtless due to the imperfect conception they have formed of what constitutes the Medium: instead of recognising in it the sum of the conditions affecting the Organism—i.e., the sum of the relations which the Organism maintains with external agencies,—they restrict or enlarge it so as to misapprehend its significance—restrict it to only a few of the conditions, such as climate, soil, temperature, &c., or enlarge it to embrace a vast array of conditions which stand in no appreciable relation to the Organism. Every one understands that an Organism is dependent on proper food, on oxygen, &c., and will perish if these are withheld, or be affected by every variation in such conditions. Every one understands that an animal which can devour or be devoured by another, will flourish or perish according to the presence of its prey or its enemy. But it is often forgotten that among external existences, all those which stand in no appreciable relation to the Organism are not properly to be included in its Medium. In consequence of this oversight we frequently hear it urged as an objection to the Evolution Hypothesis, that manifold organisms exist under the same external conditions, and that organisms persist unchanged amid a great variety of conditions. The objection is beside the question. In the general sum of external forces there are certain items which are nearly related to particular organisms, and constitute their Medium; those items which are so distantly related to these organisms as to cause no reactions in them, are, for them, as if non-existent.1 Of the manifold vibrations which

(1) At the time this was written, I had some fish ova in the course of development. Out of the same mass, and in the same vessels, all those which were supported by weed at a depth of half-an-inch from the surface, lived and developed; all those, without exception, that were at a depth of two to four inches, perished. In ordinary parlance, surely, nothing would be objected to in the phrase, "these ova were all in the same Medium;" the water was the same, the wecd the same, the vessels the same; yet some difference of temperature and carbonic acid made all the difference between life and death. Another curious fact was observed: I removed eight of these ova with active embryos, and placed them in a large watch-glass containing a solution (½ per cent.) of bichromate of ammonia. In this acid the embryos lived and were active fifty-seven hours, although other embryos placed in a similar watch glass containing pond-water, survived only forty hours. The non-effect of the acid was probably due to the same cause as nullitics the effect of certain virulent poisons when they are swallowed; but why the fish should live longer in the acid than in the simple water, I do not at all comprehend.

the ether is supposed to be incessantly undergoing, only vibrations of certain lengths and velocities affect the eye as an organ of sight; these constitute the Medium of Sight; the others are as if they were not. Only certain vibrations of the air affect the ear as Sound; to all other vibrations we are deaf; though ears of finer sensibility may detect them and be deaf to those which affect us.

"The external conditions of existence" is therefore the correct definition of the Medium. An animal may be surrounded with various foods and poisons, but if its organism is not directly adapted to them they cannot be food or poison to it. An animal may be surrounded with carnivorous rivals, but if it is not adapted to serve them as food, or is too powerful to be attacked by them, they only indirectly enter into its Medium by eating the food it would eat. The analogy is similar with anorganisms and their relation to their media. Every physical or chemical phenomenon depends on the concurrence of definite conditions: namely, the substance which manifests the change, and the medium in which the change is manifested. Alter the medium, solid, liquid, or gaseous, change its thermal or electrical state, and the phenomenon is altered. But although similar alterations in the Medium notoriously influence the Organism, yet, because a great many variations in external conditions are unaccompanied by appreciable changes in the Organism, there are biologists who regard this as a proof of Life being independent of physical and chemical laws; an error arising from their not recognising the precise nature of organic conditions.

To give greater precision to the conception of a Medium it will be desirable to adopt the distinction much insisted on by Claude Bernard, namely, 1° an External or Cosmical Medium, embracing the whole of the circumstances outside the Organism, whether directly affecting it or not, and 2° an Internal or Organic Medium, embracing the conditions inside the Organism, and in direct relation with it—that is to say, the plasma in which its tissues are bathed, by which they are nourished; to which add its temperature and electrical conditions. Bernard only includes the nutritive fluid; but inasmuch as each Organism possesses a temperature and electrical state of its own, and these are only indirectly dependent on the external temperature and electricity, and as it is with these internal conditions that the organism is in direct relation, I include them with the plasma among the constituents of the Organic Medium. According to this circum-

scription of the Medium, it represents the conditions of existence, and no other conditions: any change in the External Medium which does not disturb the Internal Medium, whether of temperature or electricity, of food or light, will of course leave the Organism undisturbed; and for the most part all the changes in the External Medium which do affect the Organism, affect it by first changing the Internal Medium. External heat or cold raises or depresses the internal temperature indirectly, by affecting the organic processes on which the internal temperature depends. We see here the rationale of acclimatisation. Unless the organism can adapt itself to the new External Medium by the readjustment of its Internal Medium, it perishes.

We are now enabled to redeem a promise made in the first part of this Essay, and furnish an answer to the very common objection respecting the apparent absence of any direct influence of external conditions. Let the objection first be stated in the words of a celebrated naturalist, Agassiz:—

"It is a fact which seems to be entirely overlooked by those who assume an extensive influence of physical causes upon the very existence of organised beings, that the most diversified types of animals and plants are everywhere found under identical circumstances. The smallest sheet of fresh water, every point of the sea shore, every acre of dry land, teems with a variety of animals and plants. The narrower the boundaries which are assigned as the primitive home of all these beings, the more uniform must be the conditions under which they must be assumed to have originated; so uniform indeed that in the end the inference would be that the same physical causes can produce the most diversified effects."

Obviously there is a complete mis-statement of the argument here; and the excess of the mis-statement appears in the following passage:—"The action of physical agents upon organised beings presupposes the very existence of those beings." Who ever doubted it? "The simple fact that there has been a period in the history of our earth when none of these organised beings as yet existed, and when, nevertheless, the material constitution of our globe and the physical forces acting upon it were essentially the same as they are now, shows that these influences are insufficient to call into existence any living being."1 Although most readers will demur to the statement that because the material constitution of our globe was "essentially the same" before and after animal life appeared, therefore there could have been no special conditions determining the appearance of Life, the hypothesis of Evolution entirely rejects the notion of organic forms having been diversified by diversities in the few physical conditions commonly understood as representing the Medium. Mr. Darwin has the incomparable merit of having en-

larged our conception of the conditions of existence so as to embrace all the factors which conduce to the result. In his luminous principle of the Struggle for Existence, and the Natural Selection which such a struggle determines, we have the key to most of the problems presented by the diversities of organisms; and the Law of Adaptation, rightly conceived, furnishes the key to all organic change.

In consequence of the defective precision with which the phrase "Medium," or its usual equivalent "physical conditions," is employed, several biological errors pass undetected. Haeckel1 calls attention to the common mistake of supposing the Organism to be passive under the influence of external conditions, whereas every action, be it of light or heat, of water or food, necessarily calls forth a corresponding reaction, which manifests itself in a modification of the nutritive process. He points out the obverse of this error in the current notion that Habit is solely due to the spontaneous action of the Organism, in opposition to the influence of external agency, as if every action were not the response to a stimulus. Corresponding with the fluctuations in the Medium there must necessarily be fluctuations of Adaptation, and I think we may safely assume that it is only when these fluctuations cease that the Adaptation becomes Habit. This is the interpretation of the phrase "Habit is second Nature," and is very different from the common interpretation which attributes it to the use or disuse of organs; as if use or disuse were a spontaneous uncaused activity.

And here we are led to consider another biological principle seldom grasped with sufficient distinctness, the relation of Function to Organ. Much confusion would be avoided by a more accurate discrimination of Functions from Properties of Tissue; but for this it would be necessary to have a precise definition of an Organ. I do not know where a satisfactory definition is to be found; which is a proof of the complexity of the question. If, therefore, I venture to propose the one to which long meditation has led me, it is with the diffidence natural to such an attempt, and with the hope that some good must issue from a more thorough discussion. A few sentences will prepare the way.

The evolution of Life is the evolution of the special from the general, the complex from the simple. An organism rises in power as it ramifies into variety. From a homogeneous organic mass a complex structure is evolved by successive differentiations. The evolution of Organs is the setting apart of certain portions of the organic mass for the exclusive performance of certain processes: the creation of special Instruments which are to subserve the general good. In the social organism we see a similar setting apart of classes of men to perform exclusively certain offices for the general welfare: the soldier fights but ceases to build, to reap, to cook, or to make clothes; the cook makes

no boots, the tailor prescribes no medicines; the priest, the thinker, and the artist, each labours in his special way, without undertaking the labours of other classes. In the simplest organisms we find no special functions. Each part does all. But while the structure is thus simple, the result is slight. For an energetic Life there is needed a complex organism. A glance at one of the higher organisms reveals the marked division of its functions: Digestion is unlike Respiration, Vision is unlike Hearing, Locomotion is unlike Prehension; these are special acts, and require special instruments. The simple organism cannot perform any one of these acts in the same efficient way. It is true that a mere lump of protoplasma exhibits the general vital phenomena of Nutrition, Reproduction, Locomotion; but how different are these phenomena both in complexity and power from those similarly named exhibited by the higher organisms! A polyp also will grasp its prey, digest it, will move and move towards the light; but to digest the various substances necessary as food for higher organisms the polyp is incompetent; equally incompetent is it to manifest the subtle and numerous actions of Vision and Prehension performed by the eye and the hand.

Hence we arrive at a twofold definition of an Organ: Physiologically, it is the special instrument by which a special Function (i.e., a particular kind of vital action) is performed; Anatomically, it is the combination of dissimilar parts by the united uses of which a special Function is effected.

It is because in our short-hand method of putting a part for the whole we commonly speak of one portion of an apparatus as if it alone did all the work, that we have accustomed ourselves to elevate into the rank of Organs parts which have no right to such a title. Thus so philosophical an anatomist as Gegenbaur follows in the beaten track, and speaks of "an organ which in one group of animals acts as a locomotive organ, in another has the function of respiration."1 Had he used the term "part" here instead of "organ," he would have avoided the implication of the common but misleading notion that Functions are not dependent upon Organs. Although this notion is a contradiction in terms, and, worse than that, is a notion which vitiates the reasoning in numerous biological questions of the highest importance, one is surprised to find how widely it is diffused, and how it may be met with, more or less disguised, in writings of considerable authority. No man would maintain that there could be an action without an agent, or that the nature of the action was not rigorously determined by the nature of the agent; but many men will be found maintaining that a Function can exist without an Organ (or an Organ without a Function), and that the nature

of the Function is not necessarily determined by the nature of the Organ. It must therefore be well worth our while to consider how such notions get accepted.

Agassiz regrets to observe that "it has almost become an axiom that identical functions presuppose identical organs. There never was a more incorrect principle, leading to more injurious consequences."1 So much for the distinctness with which the position is sometimes held. Respect for this excellent naturalist must not prevent my saying that his argument and illustrations prove him to have misapprehended the axiom, and to have confounded general functions (more correctly named "properties of tissue") with special functions. He might as well argue that the special forms of railway transport do not imply the existence of railways, because the general function of transport is also effected by stage-coaches, vans, carts, and wheel-barrows. Thus the general function (property of tissue) named Respiration is variously effected in various animals. That which is common to them all is the exchange of gases, and this is identical in all; that which is various, is the disposition of the mechanism bringing the nutritive fluid into relation with the external air; and this mechanism is identical wherever the conditions are identical, similar where they are similar, diverse where they are diverse—tracheæ, gills, or lungs. In an earlier chapter M. Agassiz maintains the same thesis on the ground of the existence of organs without functions. But there are no such organs. There are parts which have no assignable uses in the mechanism, although they must have some effects as the result of their existence. That there can be organs without functions is inconceivable, except upon a complete perversion of the meaning of an organ. This perversion is, however, not infrequent. M. Agassiz adduces in support of his position these facts:—"The teeth of the whale which never eat through the gum, and the breasts of the males of all classes of mammalia." The answer is easy. We do not ask why a violin yields none of its piercing cries when the strings are broken or slack; we do not ask why any instrument fails in its office if the necessary conditions are absent; nor should we marvel if an Organ, which equally depends on conditions, sinks from the rank of an Instrument to that of a Part with unknown uses, when the needful conditions are absent. The teeth in the gum of the (fœtal) whale are no more "organs" than the violin without strings is a "musical instrument." The breasts of males are parts, not organs; they are parts which, under suitable conditions, may yield milk, and thus serve the Function of Lactation; but under the conditions in which they are normally found in the male organism (and in the female also except at certain periods) they are as violins with slackened strings.

Spencer on a point of philosophy so fundamental as this. He is not indeed open to the charge of such a misconception as the one just noted; but he seems to have departed from his usual precision of thought and statement in his conception of Function, and to have quitted the positive point of view to range himself beside the vitalists in maintaining that Function originates Structure,1 although this leads inevitably to the conclusion that Function can exist apart from, and independently of, Structure. It is true that many passages may be quoted in which he unequivocally expresses the contrary, affirming "that the phenomena of function cannot even be conceived without direct and perpetual consciousness of the phenomena of structure— just as the laws of motion cannot be known apart from some matter moved, so there can be no knowledge of function without a knowledge of some structure performing it."2 Nevertheless, he repeatedly insists on the precedence of Function as determining Structure. "There is one fact implying that Function must be regarded as taking precedence of Structure. Of the lowest rhizopods which present no distinctions of parts, and nevertheless feed and grow and move about, Professor Huxley has remarked that they exhibit Life without Organisation."3 There is an equivoque here which arises from the practice of calling all living bodies "organisms," even those destitute of the differentiations called organs; but if we substitute the term "living body" in lieu of "organism," the equivoque will disappear, and Function no longer seem to precede Structure. Neither Mr. Spencer nor Mr. Huxley would affirm that Life can be manifested without a living body; and every living body must have a structure of some sort, unless by structure be meant some special configuration of parts. The properties of a body, whether it be simple or complex in structure, are the resultant of the properties of its components, and the vital phenomena vary with these varying components. The substance of a Rhizopod is indeed simple as compared with that of higher organisms, but is complex as compared with anorganisms; and corresponding with this simplicity of structure there is simplicity of vital function.4

The properties of steam are exhibited by the kettle on the fire no less than by the gigantic engine which animates a manufactory: but the uses of steam (the functions of the engine) vary with the varying structure, and the applications of that structure to other structures. Precisely analogous is the case of the organ and its function, in relation to the living substance of which it is a peculiar modification. We saw at the opening of this paper what vital actions were manifested by a lump of protoplasm; but these

actions are as sharply demarcated from the actions of more highly organised animals, as the phenomena of a steam engine are from those of a tea-kettle.

Mr. Spencer has nowhere defined what he means by Structure, or given a definition of Organ, and this neglect makes it difficult rightly to appreciate his view. But whether we take structure to signify the substance of the living body, or the differentiations of that substance into separate tissues and organs, in either case the actions (functions) of which this structure is the agent must be rigorously determined by it. Mr. Spencer has avowed this in declaring that the "general physiologist may consider functions in their widest sense as the correlatives of tissue." Is this true in the widest sense and not true in the narrowest? I am puzzled to find him insisting that "function from beginning to end is the determining cause of structure. Not only is this, manifestly true where the modification of structure arises by reaction from modification of function; but it is also true where a modification of structure otherwise produced apparently initiates a modification of function." Such language would be consistent were he a vitalist who believed in a Principle independent of matter which shapes matter into organic forms; but as a positive thinker he can scarcely escape the admission that since Function is the activity of the Agent (Function in the widest sense being the action of the whole Organism, and in its narrowest sense the action of the special Organ) there cannot be an activity preceding the agent. I suspect that he does not always bear in mind the distinction between General and Special Function, and consequently is led into statements at variance with the principles he professes. As far as I understand the course of his thought, it runs somewhat thus:—With the increased use of an organ its volume may be increased, its structure altered; this alteration will, by reaction, cause alterations in other organs, and thus the result of a change in the habitual activities of an animal will be an alteration in the arrangement of its parts.—We speak loosely of an organ being developed by increased activity; but this is loose speech, and investigation shows that the organ is not developed by, but accompanies the increased activity, every increment of activity being necessarily preceded by a corresponding increment of structure. This is evident à priori: the force manifested is inherent in the structure manifesting it. Thus we ought not to say "the vascular system furnishes good instances of the increased growth that follous increased function;" we ought to say, "that permits increased function." The muscle having a contractile power represented by 10, expends, we will suppose, seven units of force in its normal activity, and these are replaced by its normal nutrition. If from an extra demand upon it 9 units are expended, the muscle becomes fatigued; if 10, ex-

hausted, and it will no longer contract, the whole sum of its molecular contractility being dissipated. During all these stages the structure of the muscle—or to prevent all equivoque let us say, the substance of the muscle—has been oscillating, not indeed in any degree appreciable to the eye, but appreciable by the more decisive tests of chemical and physiological reactions. Yet inasmuch as in the ordinary course of things the waste is quickly repaired, the muscle in repose once more regains its original state, once more represents 10 units of contractility. Now let us consider what takes place when extra labour is thrown upon the muscle, when exercise causes growth. At the outset of a walking tour we may not be able to compass more than twenty miles a day, at its close we manage thirty, as our muscles have grown larger and firmer. Is it the increased activity of the function which has caused this increase of structure? In one sense, yes; but let us understand it. Had the increase of activity been temporary, we should have observed no increase of structure. But when the ordinary expenditure of 7 units rises to 9, on several successive days, this extra expenditure of tissue has had to be met by an extra repair. It is a physiological law, easily explained, that, within due limits, extra waste brings about extra repair: as the channels are widened and multiplied, the derived currents become stronger, and the encreased flow of nutrition which was temporary becomes permanent, because this increase is no longer dependent on an extra stimulus, but on an enlarged channel.1 When the channels have not become multiplied or enlarged, which must be the case whenever the extra stimulus is fluctuating and temporary, the extra expenditure is not followed by increased size of the muscle: the currents resume their old directions, no longer being diverted.

Let the social organism furnish us with an illustration. At the present moment there is a movement against the retail shopkeepers of London in favour of Co-operative Stores. The stimulus of getting better goods and cheaper, attracts the flow of custom from its old channels; and if this continue a certain time the new arrangements will be so thoroughly organised, and will work so easily, that Co-operative Stores will to a great extent supplant the retail shops. But if from any causes the stimulus slackens before this re-organisation has passed from the oscillating into the permanent stage—if the goods are not found to be superior, or the cheapness not worth the extra trouble—the old influences (aiding our indolence) which have been long and continuously at work, will cause the social organism to resume its old aspect, and the co-operative "varieties" will, disappear, or exist beside the ancient "species."

In the one case as in the other a glance at the process is enough to detect that the increase in the activity has been preceded by a

corresponding increase in the structure. The muscle has not been enlarged by extra activity, but with it. The co-operative action has grown with each additional co-operator. Looking at the cases from afar we may justly say that development has been due to function; but looking to the process we see that each increment of activity was necessarily dependent on an increment of substance. When changes of habit or adaptation are said to produce modifications in structures, this is true in as far as one modification of structure necessarily brings with it correlative modifications, the growth of one part affecting the growth of all more or less; but we must remember that to render the structure capable of new adaptations corresponding modifications must have been going on. The retail shopkeepers might securely laugh at the co-operative movement if the respectable families would not or could not become co-operant. When Mr. Spencer urges that "not only may leaf-stalks assume to a great degree the character of stems when they have to discharge the functions of stems by supporting many leaves, and very large leaves, but they may assume the characters of leaves when they have to undertake the functions of leaves," I would ask if he is not reversing the actual process? The stem cannot assume the functions of a leaf until it has first assumed the character of a leaf. The assumptions of both must be gradual, and pari passu.

The hand is an organ, its function is prehension. The performance of this function in any of its numerous applications is rigorously limited by the structure of the hand—the bones, muscles, nerves, circulating and absorbent vessels, connective tissue, fat, &c. Fatigue the nerve, and the function will be feebly performed; exhaust it, and the function ceases; diminish the action of the heart, tie an artery, or vitiate the structure of the blood, and the function will be correspondingly affected; stiffen the tendons, soften the bones, diminish the synovial fluid, or increase the fat—in short, make any alteration whatever in the structure of the hand, and an alteration is necessarily produced in its function. So rigorously is function dependent upon structure, that the hand of one man will execute actions which are impossible to another. The hand of a baby is said to be the same in structure as the hand of a man; and since the powers (functions) of the two are notoriously different, we might rashly conclude that here function was dissociated from structure The case is illustrative. In baby and man the structure is similar, not the same; the resemblance is of kind, not of degree; and the function likewise varies with the degree. The penny cannon which delights the child is similar in structure to the ten-pounder which batters down walls; and though, speaking generally, we may say that the function of both is to fire gunpowder for human ends, no one expects the penny cannon to be employed in warfare. In physiology, as in mechanics, the effect varies with the forces involved.

Mr. Spencer seems to have been led into his view by not keeping distinctly present to his mind the differences between General Function (or properties of tissue) and Special Function, or the activity of an organ. "That function takes precedence of structure," he says, "seems implied in the definition of Life. If Life consist of inner actions so adjusted as to balance outer actions—if the actions are the substance of Life, while the adjustment constitutes its form; then may we not say that the actions formed must come before that which forms them—that the continuous change which is the basis of function must come before the structure which brings the function into shape?" The separation of "actions formed" from "that which forms them" is inadmissible. An action cannot come before the agent; it is the agent in act. The continuous change, which is the basis of vitality, is a change of molecular arrangements; and the organ which gives a special direction to the vital activity, e.g., which shapes the general function of Contractility into the special function of Prehension, this organ must itself be formed before it can manifest its special function. It is true that the organs are formed by the organism, true that the general activity of living bodies must precede the special activity of any organ, as the expansions of steam must precede any steam-engine action; but the general activity depends on the general structure; and the special actions on the special structures. Although the question whether Life precedes Organisation has been often asked, it is a question mal posée. If by Organisation we are to understand not simply organic substance, but a more or less complex arrangement of that substance into separate orgaus, the question is tantamount to asking whether the simplest animals and plants have life? And to ask the question, whether Life precedes organic substance? is tantamount to asking whether the convex surface of a curve precedes the concave, or whether the motions of a body precede the body. To disengage ourselves from the complicated suggestions of such a word as Life, let us consider one of the vital phenomena, Contraction. This is a phenomenon manifested by the simple protoplasm, and by the highly differentiated form of protoplasm known as muscle. In one sense it would be correct to say that the function (contraction) precedes the organ (muscle), as the general must precede the special. But it would be absurd to. say that muscular contraction preceded the existence of muscle, and formed it. The contractions of the protoplasm are not the same as muscular contractions, no more than the hand of a baby is the same as a man's; the general property which both have in common depends on the substance both have in common; the special property which belongs to the muscle depends on its special structure. An infinite activity of the contractile protoplasm would be incompetent to form a muscle unless it were accompanied by that peculiar change in structure which constitutes

muscle. The tea-kettle might boil for ever without producing a steam-engine, or the actions of a steam-engine. That which is true of one function is true of all functions, and true of Life, which is the sum of vital activities.

This has been a long digression, but it has laid a basis for our future exposition of several questions involved in the hypothesis of Natural Selection, more especially that aspect of it which has excited the most discussion, the derivation of all organic forms from a primary cell under the two determining influences of Adaptation to changed conditions, and the preservation and increase of such adaptations in the Struggle for Existence. We have seen that Life, and all the forms of Life, result from the relation of the Organism and the Medium. Mr. Darwin has shown how this relation can only be maintained through an incessant struggle. First, the Organism has to struggle against all those external forces which are unfavourable to its constitution, when their motions do not synchronise with its motions; in this struggle it succeeds by adapting itself to them, that is, by adjusting its motions to theirs. Next it has to struggle with other organisms, to eat or be eaten by them. Thirdly, it has to struggle with rivals, and surpass them in securing the means for the preservation of its substance and the propagation of its kind. Contending against such manifold and ever-present forces of destruction, it is clear that every slight superiority which the Organism may develope will tend to bring it more and more into synchronism with external forces, cosmical and organic, and thus will be secured the "survival of the fittest," as Mr. Spencer happily phrases it.

That this light-bearing hypothesis has profoundly changed the aspect of Biology will be generally admitted. That it explains a vast body of phenomena in a way surpassing all previous hypotheses even its opponents must grant. That it explains them all, Mr. Darwin himself does not pretend. But while his opponents—as far as my knowledge reaches—have only difficulties to throw in his way, they have no hypothesis to offer which explains these difficulties, or which satisfactorily accounts for any of the phenomena; since their recourse to creative fiat or creative plan is a mere evasion of the question. Yet a final reconciliation seems not altogether hopeless. All that is indisputable in Mr. Darwin's views may be accepted without suppressing any of the facts which prevent those views from becoming universal. In these papers I shall endeavour to sketch the course such a reconciliation may take. I may be deluding myself by a natural prepossession in favour of my own conception of the Evolution Hypothesis, which has been the growth of many years' meditation, and which was very indefinite until Mr. Darwin's work came to give it shape, both by what it furnished of direct instruction, and what it suggested indirectly; but I hope to lay before the reader sufficient evidence to justify the proposed modifications. Briefly

expressed, the difference is this: Mr. Darwin assumes a community of kinship as the explanation of all organic resemblances, whereas I assume it only as the explanation of many, the others being due to similarities in the causal nexus. He assumes Adaptation and Natural Selection as the explanation of all organic diversities, whereas I assume in many cases an initial diversity in substance, and consequent diversity in epigenesis; so that we may say that many plants and animals were originally "created" on distinct foundations, and that their superstructures retain the marks of this distinction. It is here that an approach can be made towards reconciling Natural Selection with the facts which have been held to imply distinct "creations" and fixity of Species.

There are thus two questions before us, or rather two aspects of the one question. Natural Selection luminously explains how special forms arise from a given Type, as Varieties arise from Species; and in a great mass of cases it is demonstrable that special forms have arisen thus. But there is a secondary aspect of the question, namely, Are all organic forms related to each other by the bond of kinship, or are many of them only related by the similarity of their evolution? Haeckel, Mr. Darwin's thorough-going disciple, who emphatically affirms that there is no other explanation of morphological phenomena except the blood-relationship of organisms, admits that it is possible some other causes besides Natural Selection may be at work, and that these will hereafter be discovered.1 It seems to me that the Evolution Hypothesis may claim several well-known modes of operation besides that of Natural Selection, and they are implied in the existence of Analogies, such as we glanced at towards the close of the last paper on this subject.

The analogies of organic forms and functions demand a more exhaustive scrutiny than has yet been given them. Why is it that vessels, nerves, and bones ramify like branches, and why do these branches take on the aspect of many crystals? Why is it that cavities are constantly prolonged in ducts, e.g., the mouth succeeded by the œsophagus, the stomach by the intestines, the bladder by the urethra, the heart by the aorta, the ovary by the oviduct, and so on? Why are there never more than four limbs attached to a vertebral column, and these always attached to particular vertebræ? Why is there a tendency in certain tissues to form tubes, and in these tubes commonly to assume a muscular coat?2 To some of these queries an answer might be suggested which would bring them under known physical laws. I merely notice them here for the sake of emphasizing the fact that such analogies lie deeply embedded in the laws of evolution, and that what has been metaphorically called

organic crystallization, will account for many similarities in form, without forcing us to hare recourse to kinship. To take a very simple case. No one will maintain that the crystalline forms of snow have any kinship with the plants which they often resemble. Mr. Spencer has noticed the development of a wing-bearing branch from a wing of the Ptilota plumosa, when its nutrition is in excess. "This form, so strikingly like that of the feathery crystallizations of many inorganic substances, proves to us that in such crystallizations the simplicity or complexity of structure at any place depends on the quantity of matter that has to be polarised at that place in a given time. How the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small, and that they become larger when they are formed more slowly."1

It may be objected, and justly, that in the resemblance between crystals and organisms the analogy is purely that of form, and usually confined to one element, whereas between organisms there is resemblance of substance no less than of form, and usually the organisms are alike in several respects. The answer to this objection is, that wherever there is a similarity in the causal conditions there must be a corresponding similarity in the results; if this similarity extends to only a few of the conditions, the analogy will be slight; if to several, deep. But whether slight or deep we are not justified, simply on the ground of resemblance, in assuming, short of evidence, that because they are alike, two organisms are related by blood, or descended from a common ancestor.

Let us glance at a few illustrations. It has been urged, as a serious objection to Mr. Darwin's hypothesis,2 that it fails to explain the existence of phosphorescent organs in a few insects; and certainly, when one considers the widely-different orders in which these organs appear, and their absence in nearly-related forms, it is a difficulty. In noctilucæ, earth-worms, molluscs, scolopendra, and fire-flies, we may easily suppose the presence of similar organic conditions producing the luminosity; it requires a strong faith to assign Natural Selection as the cause.3 We may say the same of the electric

(1) SPENCER, Principles of Biology, II. 72.

(2) FAIVRE, Variabilité de l'Espèce, p. 15.

(3) These luminous organs would furnish an interesting digression, if space permitted it. The student is referred to the chapter in MILNE EDWARD'S Leçons sur la Physiologie et l'Anatomie Comparée, 1863, VIII. 94, sq. LEYDIG: Histologie, 1857, p. 343. KÖLLIKER: Microscopical Journal, 1858, VIII. 166, and MAX SCHULTZE, Archic für mikros. Anat. 1865, p. 124. My friend SCHULTZE was kind enough to show me some of his preparations of the organs of Lampyris splendidula, from which the drawings in his memoir were made. They reminded me of tho electric organs in fishes by a certain faint analogy, the trachea in the one holding the position of nerves in the other. I may remark, in passing, that it is not every phosphorescent animal that has distinct luminous organs. There is a lizard (Plodactylus Gecko) which occasionally becomes luminous. "A singular circumstance occurred to the colonial surgeon, who related it to me. He was lying awake in bed when a lizard fell from the ceiling upon the top of his mosqaito-curtain; at the moment of touching it the lizard became brilliantly luminous, illuminating the objects in the neighbourhood, much to the astonishment of the doctor." COLLINGWOOD, Rambles of a Naturalist, 1868, p. 169.

organs possessed by seven species of fish, belonging to five widely-separated genera. Although each species appears to have a limited geographical range, one or the other is found in almost every part of the globe. These organs occupy different positions, being now on each side of the head, now along the body, and now along the tail; and in different species they are innervated from different sources. Their intimate structure also varies; as appears from the remarkable investigations of Max Schultze.1 They cannot, therefore, be homologous. How could they have arisen? Obviously, in each case, from some analogous conditions which produced a differentiation in certain muscles. The fundamental resemblance to muscles was pointed out by Cams long ago. It has been insisted on by Leydig;2 and Owen says, "the row of compressed cells constituting the electric prism of the Torpedo offers some analogy to the row of microscopic discs of which the elementary muscle-fibre appears to consist."3 We must not, however, forget that these resemblances are merely such as suggest that the electric organ is a differentiation of the tissue which elsewhere is muscular, and that Dr. Davy was justified in denying the organ to be muscular.4 That it is substituted for muscle cannot be doubted. Now, although we are entirely ignorant of the conditions which cause this differentiation of muscles into electric organs, we can understand that, when once such a development had taken place, if it in any way profited the fish in its struggle for existence, Natural Selection would tend to its further increase and propagation. So far Mr. Darwin carries us with him. But we decline proceeding further. The development of these organs in fishes, so widely removed, does not imply an ancestral community. The similarity in concurrent conditions is quite enough to account for the phenomenon. This, with his accustomed candour, Mr. Darwin admits. "If the electric organs," he says, "had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would be specially related to each other. Nor does Geology at all lead to the belief that formerly most fishes had electric organs which most of their modified descendants have lost."

It may seem strange that he should thus urge a difficulty against his hypothesis when it could be avoided by the simple admission that even among nearly-allied animals great differences in develop-

ment are observable, and the electric organs might be ranged under such diversities. But Mr. Darwin has so thoroughly wrought out his scheme, that he foresees most objections, and rightly suspects that if this principle of divergent development be admitted, it will cut the ground from under a vast array of facts which his hypothesis of descent requires. For observe: if we admit that differentiations so marked, and organs so important, can have arisen from organic conditions which spontaneously presented themselves in very different animals, and therefore that these organs, although so closely resembling each other, are not due to ancestral influence, we can hardly refuse to extend to the whole organism what we have admitted of a particular organ; and thus the admission of the spontaneous evolution of closely-resembling organs carries with it the admission of the spontaneous evolution of closely-resembling organisms; that muscular tissue should, under certain similar conditions, develope into electric organs, is but one case of the law that organic substance, under certain similar conditions, will develope, into organisms closely resembling each other.

It is to be remarked that Mr. Darwin fixes his attention somewhat too exclusively on the Adaptations which arise during the struggle for existence, and to that extent neglects the laws of organic growth; just as Lamarck too exclusively fixed his attention on the influence of external conditions and of wants. Not that Mr. Darwin can be said to overlook the organic laws; he simply under-estimates the part they play. Occasionally he seems arrested by it, as when instancing the "trailing palm in the Malay Archipelago, which climbs the loftiest trees by the aid of exquisitely constructed hooks, clustered around the ends of the branches, and this contrivance no doubt is of the highest service to the plant; but as there are nearly similar hooks on many trees which are not climbers, the hooks on the palm may have arisen from unknown-laws of growth, and have been subsequently taken advantage of by the plant undergoing further modification and becoming a climber."

Again I come round to the position from which I started, that the resemblances traceable among animals are no proof of kinship; even a resemblance so close as to defy discrimination would not, in itself, be such a proof. The absolute identity of chalk in Australia and in Europe is a proof that there was absolute identity in the formative conditions and the constituent elements, but no proof whatever that the two substances were originally connected by genesis. In like manner the similarity of a plant or animal in Africa and Europe may be due to a common kinship, but it may also be due to a common history. It is barely conceivable that the history, from first to last, would ever be so rigorously identical in two parts of the globe as to produce complex identical forms in both; because any diversity

either in structure or external conditions may be the starting-point of a wide diversity in subsequent development; and the case of organic combinations is so far unlike the inorganic, that while only one form is possible to the latter (chalk is either formed or not formed) many forms are possible to organic elements. But although identities so close as those of Species and Genera are not readily conceivable as the products of an identical history in different quarters of the globe, it is not only conceivable, but is eminently probable, that Orders and Classes have no nearer link of relationship than is implied in their community of organic substance and their common history. The fact that there is not a single mammal common to Europe and Australia is explicable, as Mr. Darwin explains it, on the ground that migration has been impossible to them; but it is also explicable on the laws of evolution—to have had mammals of the same species and genera would imply a minute coincidence in their history which is against the probabilities. Again, in the Oceanic Islands there are no Batrachians. But there are Reptiles, and these conform to the reptilian type. Mr. Darwin suggests that the absence of Batrachia is due to the impossibility of migration, their ova being destroyed by salt water. But may it not be due to the divergence from the reptilian type, which was effected elsewhere, not having taken place in these regions? When we find Tin in Prussia and Cornwall, and nowhere else in Europe, must we not conclude that in these two countries, and nowhere else, a peculiar conjunction of conditions caused this peculiar evolution?

The question at issue is, Are the resemblances observable among organic forms due to remote kinship, and their diversities to the divergencies caused by adaptation to new conditions? or are the resemblances due to similarities and the diversities to dissimilarities in the history of organic beings? Are we to assume one starting-point and one centre of creation, or many similar starting-points at many centres? So far from believing that all plants and animals had their origin in one primordial cell, at one particular spot, from which descendants migrated and became diversified under the diverse conditions of their migration, it seems to me more consistent with the principle of Evolution to admit a vast variety of origins more or less resembling each other; and this initial resemblance will account for the similarities still traceable under the various forms; while the initial differences becoming intensified by development under different conditions will yield the diversities. The evolution of organisms, like the evolution of crystals, or the evolution of islands and continents, is determined by laws inherent in the substances evolved, and by their relations to the medium in which the evolution takes place. This being so, we may à priori affirm that the resultant forms will have a community strictly corresponding with the identity

of the substances and their conditions of evolution, together with a diversity corresponding with their differences in substance and conditions. It is usually supposed that the admission of separate "centres of creation" is tantamount to an admission of "successive creations" as interpreted by the majority of those who invoke "creative fiats." But the doctrine of Evolution, which regards Life as making its appearance consequent upon a definite concurrence of conditions, and regards the specific forms of Life as the necessary consequences of specific circumstances, must also accept the probability of such conditions occurring at different times and in different places. Upon what grounds, cosmical or biological, are we to assume that on only one microscopic spot of this developing planet such a group of conditions was found—on only one spot a particle of protein substance was formed out of the abundant elements, and under conditions which caused it to grow and multiply, till in time its descendants overran the globe? The hypothesis that all organic forms are the descendants of a single germ, or of even a few germs, and are therefore united by links of kinship more or less remote, is not more acceptable than the hypothesis that all the carbonates and phosphates, all the crystals, and all the strata found in different parts of the globe are the descendants of a single molecule, or a few molecules; or,—since this may seem too extravagant,—than that the various maladies which afflict organic beings are, in a literal sense, members of families having a nearer relationship than that of being the phenomena manifested by similar organs under similar conditions—a conception which might have been accepted by those metaphysical pathologists who regarded Disease as an entity. Few philosophers have any hesitation in supposing that other planets besides our own are peopled with organic forms, though, from the great differences in the conditions, these forms must be extremely unlike those of our own planet. If separate worlds, why not separate centres? The conclusion seems inevitable that wherever and whenever the state of things permitted that peculiar combination of elements known as organic substance, there and then a centre was established—Life had a root. From these roots closely resembling each other in all essential characters, but all more or less different, there have been developed the various stems of the great tree. Myriads of roots have probably perished without issue; myriads have developed into forms so ill-adapted to sustain the fluctuations of the Medium, so ill-fitted for the struggle of existence, that they became extinct before even our organic record begins; myriads have become extinct since then; and the descendants of those which now survive are like the scattered companies and regiments after some terrific battle.

But to make this clear we must ask the reader's patience to follow us still further in our survey of the phenomena.

MR. DARWIN'S HYPOTHESES.

CONCLUSION.

THERE seems to me only ono alternative logically permissible to the Evolution Hypothesis, namely, that all organic forms have had either a single origin, or numerous origins; in other words, that a primordial cell was the starting-point from which all organisms have been successively developed, or that the development issued from many independent starting-points, more or less varied. This is apparently not the aspect presented by the hypothesis to the generality of its advocates; they seem to consider that if all organic forms are not the lineal descendants of one progenitor, they must at any rate be the descendants of not more than four or five. The common belief inclines to one. Mr. Darwin, whose caution is as remarkable as his courage, and whose candour is delightful, hesitates as to which conclusion should be adopted: "I cannot doubt," he says, "that the theory of descent, with modifications, embraces all the members of the same class. I believe that animals have descended from, at most, only four or five progenitors, and plants from an equal or lesser number. Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide."

Although he thus checks the rein, the mere suggestion of all organic life having had its origin in a single cell, has thrown the theological world into a flutter, whilst it has flattered the speculative tastes of another class. Let us try to disengage ourselves from all terror and from all desires for à priori simplicity, and examine the reach of the evidence. Perhaps the evidence may lead to the conclusion that if a single origin be relinquished, many origins must be admitted, since the difficulties of reducing all the forms to one—difficulties which lightly hamper abstract speculation, but which press heavily on the inquirer entering the field of concrete observation—are not to be escaped by the small concession of four or five origins. To arrange all the forms in groups, diverging from a common centre, as branches and branchlets from a single stem, may be, as a subjective conception, more or less admissible; but such an arrangement implies certain assumptions, and these are given up by the admission of a few independent origins. The reasons for admitting more origins than one are good biological reasons, but they carry with them a fatal necessity of extension—the door once opened for the entrance of a few, is thrown wide open for the entrance of many.

ciliation might be effected between those who maintain the fixity of species and those who maintain the variability of species. We throw a bridge across the gulf when we establish the multiplicity of origins; we do not indeed lessen the distance between the two antagonistic conceptions of the world (Monistic and Dualistic), with which the views respecting species are allied, but we may bring about on the part of the evolutionists a concession of some cardinal positions held by their antagonists, and so remove the great obstacles to the general acceptance of Evolution. That I incline to a multiplicity of origins, the reader has already seen; and it now remains for me briefly to justify that position. The view itself may be thus indicated. In lieu of conceiving all organic resemblances as the inheritance from a common source, and all the diversities as divergences from that source, it is more consistent to assume that the resemblances, though very often due to kinship, are very often due to the similarities which would necessarily exist in organisms quite independent of each other; and the diversities as often due to the initial differences existing in these origins, as to the differences in life-history. That is to say, Organic Substance, though evolved at a thousand different points of the globe, would necessarily have a great similarity underlying its varieties; and the development of this substance, following uniform laws, and under conditions generally similar, would exhibit a life-history similar in all essential characters; but from the first there must have been differences, and the marked diversities in organic forms have thereby been determined.

Both of the hypotheses here contrasted being beyond the reach of proof we have only to ask which of them best groups facts and inferences into unforced accordance. Towards the close of my last paper I suggested that nothing warrants the assumption of Life having originated solely in one microscopic lump of protoplasm on one point of our earth's surface; on the contrary, it is more probable that from innumerable and separate points of this teeming earth, myriads of protoplasts sprang into existence, whenever and wherever the conditions of the formation of organic substance were present. It is probable that this has been incessantly going on, and that every day new protoplasts appear, struggle for existence, and serve as food for more highly organized rivals; but whether an evolution of the lower forms is, or is not, still going on, there can be no reluctance on the part of every believer in Evolution to admit that when organic substance was first evolved, it was evolved at many points. If this be so, the community observable in organic substance, wherever found, may as often be due to the fact of a common elementary composition as to the fact of inheritance. If this be so we have a simple explanation both of the fundamental resemblances which link all organisms together, and of the characteristic diversities which

separate them into kingdoms, classes, orders, genera, species. The resemblances are many, and close, because (1) the forms evolved had a similar elementary composition, and (2) their stages of evolution were determined by similar conditions; the diversities are many, because (1) the forms evolved had from the first some diversities in elementary composition, and (2) their stages of evolution have been determined under conditions, which, though similar in general, have varied in particulars. Indeed, there is no other ground for the resemblances and differences among organic beings than the similarities and dissimilarities in their Substance and History; and, whether the similarities are due to blood-relationship, or to analogy, the results are the same. There is something seductive in the supposition that Life radiated from a single centre in ever-increasing circles, its forms becoming more and more various as they came under more various conditions, until at last the whole earth was crowded with diversified existences. "From one cell to myriads of complex organisms, through countless æons of development," is a formula of speculative beauty, but I cannot bring myself to accept it; and I think that a lingering influence of the tradition of a creative fiat may be traced in its conception. May we not rather assume that the earth at the dawn of Life was like a vast germinal membrane, every slightly diversified point producing its own vital form; and these myriads upon myriads of forms—all alike and all unlike—urged by the indwelling forces of development, struggled with each other for existence, many failing, many victorious, the victors carrying their tents into the camping ground of the vanquished? This metaphor of a germinal membrane is suggested by the analogy traceable between the evolution of organic forms and the evolution of anatomical elements—of which more anon; but it must, of course, be understood as only a metaphor. The point raised is the immense improbability of organic substance having been evolved only in one microscopic spot; if it were evolved at more than one spot, and under slightly varying conditions, there would necessarily have arisen in these earliest formations the initial diversities which afterwards determined the essential independence and difference of organisms.

Let us for a moment glance at the resemblances and diversities observable in all organisms. All have a common basis, all being constructed out of the same fundamental elements: carbon, hydrogen, nitrogen, and oxygen; these (the organogens, as they are named), with varying additions of some few other elements, make up what we know as Organic Matter, vegetal and animal. Another peculiarity all organisms have in common, namely, that their matter is neither solid nor liquid, but viscid. Beside this community of Substance we must now place a community of History. All organisms

grow and multiply by the same process; all pass through metamorphic stages ending in death; all, except the very simplest, differentiate parts of their substance for special uses, and these parts (cilia, membranes, tubes, glands, muscles, nerves), have similar characters in whatever organism they appear, and their development is always similar, so that the muscles or nerves of an intestinal worm, a lobster, or a man, are in structure and history essentially alike. When, therefore, we see that there is no biological character of fundamental importance which is not universal throughout the organic world, when we see that in Structure and in History all organisms have a community pervading every variety, it is difficult not to draw the conclusion that some hidden link connects all organisms into one; and when, further, it is seen that the most divergent forms may be so arranged by the help of intermediate forms only slightly varying one from the other, that the extreme ends, the monad and the man, may be connected, and a genealogical tree constructed, which will group all forms as modified descendants from a single form, the hypothesis that kinship is the hidden link of which we are in search becomes more and more cogent.

But now let the other aspect be considered. If there is an unmistakeable uniformity, there is also a diversity no less unmistakeable. The chemical composition of organic substances is various. Unlike inorganic substances, the composition of which is rigorously definite, organic substances are, within narrow limits, variable in composition. With the four organogens, which are constant, there are elements present in some organisms which are absent in others, and when present, variable in quantity. Albumen is somewhat different in blood, muscle, and white of egg,—it differs in the muscles of different animals; and all other organic substances will vary as much as one per cent in each of their constituents. We speak, and speak truly, of nerve-tissue, or muscle-tissue, being the same throughout the animal kingdom; yet it should be understood that these tissues are "the same," as all animals are the same—the fundamental resemblances not excluding immense diversities. I have recently been startled at the very great variations exhibited by the nerve-tissue of different classes and families, not only in its histological details, but in its physical and chemical characters; and from the investigations of Valenciennes and Fremy, it appears that the chemical composition of muscle differs greatly in the crustacea and the vertebrata; very noticeable is the fact that the crustacean muscle contains no appreciable quantity of phosphate of potass or oleophosphoric acid, and mere traces of creatin and creatinin.

I pass over the resemblances and differences observed in the earliest stages of development, marked as they are, and direct attention to the fact, that down at what must be considered the

very lowest organic region, we meet with differences not less striking than those met with in the highest, we find structures (if structures they may be called), which cannot be affiliated, so widely divergent is their composition. The structureless vibrio, for example, is not only capable of living in a medium destitute of oxygen, but is, according to M. Pasteur, actually killed by oxygen; whereas the equally simple bacteria can no more dispense with oxygen than other animals can. Consider for a moment the differences implied in the fact that one organism cannot even form an enveloping membrane to contain its protoplasm, whereas another contrives to secrete an exquisite shell; yet between the naked Rhizopod and the shelled Rhizopod our lenses and reagents fail to detect a difference. One Monad can assimilate food of only one kind, another Monad assimilates various kinds.1 What a revelation of chemical differences appears in the observations of M. Pasteur respecting the vibrio and bacteria, in a fermentescible liquid—the former preparing the putrid fermentation which the latter completes! We cannot doubt that some marked difference must exist between the single-celled organism which produces alcoholic fermentation, and that which produces acetic fermentation, and that again which produces butyric fermentation; and if we find distinctions thus established at the lowest region of the organic series, we need not marvel if the distinctions become wider and more numerous as the series becomes more diversified. The structure and development of an organism are dependent on the affinities of its constituent molecules, and it is a biological principle of great importance which Mr. Paget insists on, when he shows how "the existence of certain materials in the blood may determine the formation of structures in which they may be incorporated."2Any initial diversity may thus become the starting point of a considerable variation in subsequent evolution. Thus, supposing that on a given spot there are a dozen protoplasts closely resembling each other, yet each in some one detail slightly varying; if this variation is one which, by its relations to the external medium, admits of a difference in the assimilation of materials present in the medium, it may be the origin of some new direction in development, and the ultimate consequence may be the formation of a shell, an internal skeleton, a muscle, or a nerve. Were this not so, it would be impossible to explain such facts as that chitine is peculiar to the Articulata, cellulose to Molluscoida, carbonates of lime to Mollusca and Crustacea, and phosphates to Vertebrata—all assimilated from the same external medium. But we see that from this medium one

(1) On the nutrition of Monads, see the remarkable memoir by CIENKOWSKI, in the Archiv für mikros. Anatomie, i. 221, sq.

(2) PAGET: Lectures on Surgical Pathology, edited by TURNER, 1865, p. 19. The reader is advised to give careful consideration to this and the many other valuable suggestions luminously set forth in Mr. Paget's work.

organism selects the materials which another rejects; and this selection is determined by the nature of the structure which assimilates only those materials it is fitted to assimilate. We hear a great deal of Adaptation determining changes of structure and function, and are too apt to regard this process as if it were not intimately dependent on a corresponding statical change. By no amount of external influence which left the elementary composition of the structure unchanged, could an organism with only two tissues be developed into an organism with three or four. By no supply or stimulus, could an animal incapable of assimilating peroxide of iron acquire red blood corpuscles, although it might have the iron without the corpuscles; nor could an oyster form its shell unless capable of assimilating carbonate of lime. For myriads of years, in seas and ponds, under endless varieties of external conditions, the amœba has lived and died without forming a solid envelope, although the materials were abundant, and other organisms equally simple have formed envelopes of infinite variety. In all the seas, and from the earliest ages, zoophytes have lived, and assumed a marvellous variety of shapes and specialization of functions; but although some of them have acquired muscles, none have acquired nerves, none bone. Ages upon ages rolled on before fishes were capable of forming bone, and thousands are still incapable of forming it, though living in the same waters as the osseous fishes.

"Looking to the dawn of life," says Mr. Darwin (repeating an objection urged against his hypothesis), "when all organic beings, as we imagine, presented the simplest structure, how could the first steps in advancement, or in the differentiation and specialisation of parts have arisen? I can make no sufficient answer; and can only say that, as we have no facts to guide us, all speculation would be baseless and useless."

Where Mr. Darwin hesitates, lesser men need extra caution; but I must risk the danger of presumption, at least so far as to suggest that while an answer to this question is difficult on the dynamical view of Evolution (that which regards Function as determining Structure), it is less difficult on the statico-dynamical view propounded in these pages; the difficulty which besets the explanation when all the manifold varieties of organic forms are conceived as the successive divergences from an original starting-point, is much less when a variety of different starting-points is assumed, in each of which some initial diversity prepared the way for subsequent differentiations; just as we know that between the ovum of a vertebrate and the ovum of an invertebrate, similar as they are, there is a diversity which manifests itself in their subsequent evolution. If Function is determined by Structure, and evolution is the product of the two, it is clear that the different directions in the lines of develop-

ment will have their origin in structural differences, and not in the action of external circumstances, unless these previously bring about a structural change. The action of the Medium on the Organism is assuredly a potent factor which Biology cannot ignore: but the Organism itself is a factor, and according to its nature the influence of the Medium is defined. What marvellous developments are possible to a very slight germ is instructively illustrated in the history of Astronomy; considered in itself the invention of Trigonometry was of small importance, yet had there arisen no Hipparchus to devise this method of measuring angles, all that we pride ourselves on in the science of Astronomy would have been impossible. The appearance of Trigonometry was like the appearance in the animal organism of a ganglionic cell and nerve fibre, the starting-point of new and vast possibilities.

What is the theoretic significance of the differences here insisted on? We may interpret them either as evidence that the organic world has had a vast variety of origins, or as evidence of successive gradations in the evolution of the one primordial protoplast. Seeing how the marked differentiations successively appear in the apparently homogeneous ovum of the highest animal, how muscle and nerve and gland are successively wrought out of a layer of cells which contained no traces of them, there seems no objection à priori to the inference that in the development of the animal series from a protoplast, differentiations of structure may have successively appeared. No objection à priori, but many à posteriori! The facts, as we know them, do not point to a single origin, but to many; and it is these facts which sustain the belief in a "fixity of species" and in "creative fiats," although the doctrine of Evolution may equally claim them.

Quitting for a moment the track of this argument, let us glance at the resemblances and differences observable in Plants and Animals, because most people admit that these have separate origins. The resemblances are scarcely less significant than those existing among animals. Both have a similar basis of elementary composition; not only are both formed out of similar protein compounds, protoplasts with similar properties, but in both the first step from the protoplasm to definite structure is the Cell. And the life of this Cell is remarkably alike in both, its phases of development being in many respects identical; nay, even such variations as obtain in the cell-membranes are curiously linked together by a community in the formative process.1 In both Plants and Animals we find individuals constituted—1st, by single cells; 2nd, by groups of cells undistinguishable among each other; and 3rd, by groups of differentiated cells. In both we find colonies of individuals leading a common life. In both the processes of Nutrition and Reproduction are essentially similar; both propa-

gate sexually and asexually; both exhibit the surprising phenomena of parthenogenesis and alternate generations. In both there are examples of a free-roving embryo which in maturity becomes fixed to one spot, losing its locomotive organs and developing its reproductive organs. In both the development of the reproductive organs is the climax which carries Death. So close is the analogy between plant-life and animal-life, that it even reaches the properties usually held to be exclusively animal; I mean that even should we hesitate to accept Cohn's discovery of the muscles in certain plants,1 we cannot deny that plants exhibit Contractility; and should we refuse to interpret as Sensibility the phenomena exhibited by the Sensitive Plants, we cannot deny that they present a very striking analogy to the phenomena of Sensibility exhibited by animals.

It is unnecessary to continue this enumeration, which might easily be carried into minute detail. A chapter of such resemblances would only burden the reader's mind, without adding force to the conclusion that a surprising community in Substance and Life-history must be admitted between Plants and Animals. This granted, we turn to the differences, and find them to be no less fundamental and detailed. Chemistry tells us nothing of the differences in the protoplasms from which animals and plants arise; but that initial differences must exist is proved by the divergence of the products. The vegetable cell is not the animal cell; and although both plants and animals have albumen, fibrine, and caseine, the derivatives of these are unlike. Horny substance, connective tissue, nerve tissue, chitine, biliverdine, creatine, urea, hippuric acid, and a variety of other products of evolution or of waste, never appear in plants; while the hydrocarbons so abundant in plants are, with two or three exceptions, absent from animals. Such facts imply great differences in elementary composition; and this result is further enforced by the fact that where the two seem to resemble, they are still unlike: the plant protoplasm forms various cells, but never forms an epithelial-cell or nerve-cell; fibres, but never a fibre of elastic tissue; tubes, but never a nerve tube; vessels, but never a vessel with muscular coatings; solid "skeletons," but always from an organic substance (cellulose), not from phosphates and carbonates. In no one character can we say that the plant and the animal are identical; we can only point throughout the two kingdoms to a great similarity accompanying a radical diversity.

Having brought together the manifold resemblances, and the no

(1) FERDINAND COHN: Die contractile Gewibe im Pflanzenreich, 1862. By a series of numerous well-devised experiments, Cohn found that in the stamen of the centauria a tissue exists which is excitable by the same stimuli as muscle is, and which reacts like muscle, describing a similar curve when excited, and, after reaching its maximum, relaxing. Like the muscle it becomes fatigued by repeated contraction, and recovers its powers by repose. Like the muscle it may be rendered tetanic.

less marked diversities, we must ask what is their significance? Do the resemblances imply a community of origin, an universal kinship? If so, the diversities will be nothing more than the divergences which have been produced by variations in the Life-history of the several groups. Or—taking the alternative view—do the diversities imply radical differences of origin? If so, the resemblances will be nothing more than the inevitable analogies resulting from Organic Substance being everywhere somewhat similar in composition, and similar in its phases of evolution. To state the former position in the simplest way, we may assume that of two masses of protoplasm having a common parentage, one, by the accident of assimilating a certain element not brought within the range of the other, thereby becomes so differentiated as to form the starting-point of a series of evolutions widely divergent from those possible to its congener; and at each stage of evolution the introduction of a new element (made possible by that stage) will form the origin of a new variation. It is thus feasible to reduce all organic forms to a primordial protoplasm, in the evolutions of which successive differentiations have been established. On the other hand, it is equally feasible to assume that the existence of radical differences must be invoked to account for the possibility of the successive differentiations. Which of these hypotheses best accords with what we know?

As Goethe pointed out in his remarkable review of the controversy between Cuvier and Geoffrey St. Hilaire, there are two orders of intellect—the analytical, which attaches itself more readily to differences, and the synthetical, which attaches itself more readily to resemblances; the one is apt to lose its hold of the law in grasping at all the details, the other is apt to lose sight of the details in trying to trace out the law. Both have their strength and both their weakness. Laws are but organised facts; facts are the elements of laws. Every thinker who has attempted to work out a theory, with a real desire to bring all the known facts under it, will acknowledge the increasing hesitancy and the moments of despair which oppressed him more and more as his mind became saturated with the details—that speculative play of thought, which was so easy at a distance from the details, becoming more and more difficult in their presence; the facts insisting on recognition crowd upon him, they are felt to be significant, yet their significance is unascertained; to disregard them would be perilous, yet to give them their due place seems hopeless. Do we not observe in the majority of writers on the "Origin of Species" a tendency to bring forward the resemblances, undervaluing the differences, or else to bring forward the differences, as if these alone were significant? The immense services which the pursuit of resemblances has in the hands of the transcendental anatomists rendered to Biology, by the disclosure of relationships

where they were least obvious, must not blind us to the fact that the hunt after resemblances has led to much mistaken speculation; and with reference to the topic now before us, it may be urged, that although by attaching ourselves to the points of community, in disregard of the diversities, we may make it appear that all animals have a common parentage, and that plants and animals are merely divergent groups of the same prototype, a rigorous logic will force us onwards, and compel us to admit that a kinship no less real unites the organic with the inorganic world. For upon what principle are we to pause at the cell or protoplasm? If by a successive elimination of differences we reduce all organisms to the cell, we must go on and reduce the cell itself to the chemical elements out of which it is constructed; and inasmuch as these elements are all common to the inorganic world, the only difference being one of synthesis, we reach a result which is the stultification of all classification, namely, the assertion of a kinship which is universal. We must bear in mind that all things may be reduced to a common root by simply disregarding their differences. All things are alike when we set aside their unlikeness.

Suppose, for the sake of illustration, we regard an Orchestra in the light of the Development Hypothesis. The various instruments of which it is composed have general resemblances and particular differences, not unlike those observable in various organisms; and as we proceed in the work of classification we quickly discover that they may be arranged in groups analogous to the Sub-kingdoms, Classes, Orders, Genera, and Species of the organic world. Each group has its cardinal distinction, its initial point of divergence. All musical instruments resemble each other in the fundamental character of producing. Tone by the vibrations of their substance. This may be called their organic basis. The first marked difference which determines the character of two sub-kingdoms (namely, instruments of Percussion and Wind instruments) arises from a difference in the method of impressing the vibrations; and the grand divisions of these subkingdoms arise from the nature of the vibrating substances. Each type admits of many modifications, but the primary distinction is ineffaceable. We can conceive the Pipe modified into a Flute, a Flageolet, a Clarionet, a Hautbois, a Bassoon, or a Fife, by simple accessory changes; to modify the Pipe into a Trumpet, and thus produce the peculiar timbre of the trumpet, would be impossible except by the substitution of a new material; by replacing the wood with metal we may adhere to the old Type, but we have created a new Class. (Attention is requested to this point, because the current views respecting the transmutation of tissues, which seem to lend a decisive support to the hypothesis of the transmutation of species, are very commonly vitiated by the confusion of transformation with substitution. No anatomical element is trans-

formed into another specifically different—an epithelial-cell into a nerve-cell, for instance—but one anatomical element is frequently substituted for another.) To convert the Pipe or the Trumpet into a Violin or a Drum would be impossible. We can follow the modifications of a Tambourine into a Drum or Kettledrum, but no modifications of these will yield the Cymbals. That is to say, the vibrating materials—wood, metal, parchment, and the combination of wood and strings—have peculiar properties, and the instruments formed of such materials must necessarily from the very first belong to different groups, each subdivision of the groups being dependent on some characteristic difference in methods of impressing the vibrations, or in the materials. Although all musical instruments have a common property and a common purpose, we do not regard them as transformations of one primitive instrument; their kindred nature is a subjective conception: their analogies are numerous and close, but we know their origin. It is obvious that men being pleased by musical tones, have been led by their delight to construct instruments whenever they have discovered substances capable of musical vibrations, or methods of impressing such vibrations. By substituting the bow for the plectrum or the fingers, they may have changed the Lyre into the Violin, Viola, Violoncello, and Bass. (It seems historically probable that the real origin of the Violin class was an instrument with one string played on by a bow.) By grouping together Pipes of various sizes they got the Pan-pipes; by substituting metal and enlarging the blowing-apparatus they got the Organ. By beating on stretched parchment with the finger they got the Tambourine and Tom-Tom; by doubling this and using a stick they got the Drum. By beating metal with metal they got the Cymbals; by beating wood with wood they got the Castanets.

The application of this illustration is plain. Just as a Wind-instrument is incapable of becoming a Stringed-instrument, so a Mollusc, with all its muscles unstriped and its nervous system unsymmetrical, is incapable of becoming a Crustacean, with all its muscles striped and its nervous system symmetrical. Indeed, there are probably few biologists of the present day who imagine the transmutation of one kind into the other to be possible; but many biologists assume that both may have been evolved from a common root. The point is beyond proof; yet I think there is a greater probability in the assumption that both were evolved from different roots. At any rate, one thing is certain; a divergence could only have been effected by a series of substitutions; and the question when and how these substitutions took place is unanswerable: one school believes them to have been creative fiats, the other school believes them to have been transmutations.

In the remarks I have made on the hypothesis of Natural Selection as an explanation of organic diversities, the reader must not see a

disposition to undervalue the immense part played in Biology by what will hereafter be known as the Lex Daruiniana; but only a desire to leave the doctrine of Evolution free to include the Struggle for Existence among other factors. There can be no doubt that Natural Selection (aided by some minor laws—as, for example, Moritz Wagner's "Law of Migration"),1 while it gives a sudden precision to parts of the evolution doctrine that were very vague, also gives a satisfactory explanation of the origin of many diversities; whether it explains them all is another matter. As I conceive it, the very existence of initial diversities was necessary to permit Natural Selection to come into play; instead of creating these, Selection used them. Holding firmly to the doctrine of Evolution, we may assert that wherever and whenever the conditions are identical the lines of direction followed by development will coincide, and the results will be identical; wherever the conditions are similar the results will be similar; wherever the conditions are different the lines of direction will be divergent, and the results be dissimilar. It is only necessary to add that under conditions of development are included the nature of the organism and its relations to the external medium—in a word, the whole Life-history.

Although I have not touched upon any of the various topics which arise in the application of Mr. Darwin's principles, my remarks have already so trespassed on the comparatively scanty space which the REVIEW can afford, that they must here come to a close. But to complete, at least, the outline of my programme, a few words must be given to the second of the hypotheses put forth by Mr. Darwin, the one which he has named Pangenesis. It is advanced only as a provisional conception grouping together a multitude of facts, and, as such, it will be generally admitted to surpass all previous attempts in the same direction. Even were it presented with a far less suggestive array of unforced interpretations, biologists would be ill advised if they failed to treat it with peculiar circumspection, for they might be assured that a mind so large and so conscientious as Mr. Darwin's—wide-sweeping in its circuit and patient in research—would be misled by no hasty glance, no precipitate generalisation, to seek the temporary effect of a fleeting paradox, or a showy speculation. He has, doubtless, like other theorists, sometimes yielded to the bias of a foregone conclusion; but candid judges will admit, even when they reject his views, that few theorists have been at once so daring and so circumspect. It is, therefore, simple prudence which suggests that an hypothesis carefully worked out by such a thinker should be criticised with something of a corresponding hesitation, and not dismissed if it fails to carry conviction with it at once. Especially should we guard

against assigning much value to any objections which may readily present themselves. This is a common failing; yet a moment's reflection will show that there is something too naïve in the supposition that objections which occur so readily to us have not also occurred to him, and been rejected by him as of no weight. Men who, perhaps, have never meditated on the problem until it was presented in his pages, have the quiet arrogance of assuming that they detect fatal objections where he, who has spent months of research, failed to see them: arrested by what lies on the surface, they imagine it has escaped his observation, because it failed to arrest him who has looked below the surface. I do not, of course, mean that out of respect even to Mr. Darwin's well-merited authority we should disregard objections; only we should bear in mind the probability that he has already considered them, and that if he passes them by we should hesitate long before concluding them to be fatal.

The need for some such hypothesis has long been felt. Every one, as he remarks, must wish to explain, even if imperfectly, how it is possible for a bodily or mental characteristic which distinguished a parent to reappear in the offspring—how the peculiarity of an ancestor suddenly reappears in a descendant after lying dormant through generations; and this peculiarity may be a feature, a predisposition to disease, a monstrous deviation from the typical form, a perverted instinct or a glorious gift. Nor is this explicable by "coincidence;" the phenomenon is producible under known conditions; thus, the colour of the rock pigeon—from which all our breeds of pigeons are descendants—is blue; and if we pair a black pigeon with a white pigeon the offspring are not black and white or grey pigeons, as might be expected, but pigeons blue like their ancestors; the two colours, black and white, from some unascertained cause, refuse to blend here, and the result of this is that the blue colour, which for generations has lain dormant in black and white pigeons, is liberated from those causes which originally modified it, and reappears.

Further, every one wishes to frame some intelligible explanation of the effects of increased or decreased use in limb or muscle being transmitted to the offspring; and why rudimentary or supplementary organs are inherited; why the sperm-cell in plant and animal can not only carry to the offspring many peculiarities of the male, but can so markedly affect the reproductive organs of the female, that her subsequent offspring, by other males, inherit particularities which are not traceable in their own male progenitors, but are distinctly traceable in the male progenitor of their elder brothers and sisters: thus, if a thorough-bred female be once crossed with a mongrel or male of distinct breed, all the subsequent progeny with males of her own breed will bear the traces of the earlier mixture. Among the

many marvels of generation this has been considered peculiarly puzzling, and among the many strange facts which have been authenticated few are more decisively authenticated than this.

Not only the phenomena of Heredity, but also the seemingly less marvellous phenomena of repair of injuries and reproduction of lost limbs (which have already been touched on in these papers), are brought under Mr. Darwin's hypothesis, and receive a satisfactory explanation, on the assumption that the hypothesis itself is satisfactory. Let us endeavour to state it briefly:—

An organism is constructed out of a multitude of units (except in those simplest organisms lying at the basis of the series), which units are, in the current theories of the day, regarded as cells. Each unit, or cell, leads its own life, has its own stomach, is an independent member of the great community; and each reproduces its kind. Mr. Darwin assumes that over and above this means of increase, by which cells are multiplied and tissues grow, there is another infinitesimal generation by which each cell (and, indeed, each part of a cell) throws off atoms or "gemmules." I have added the clause in the parenthesis, because, although not stated by Mr. Darwin, it is a necessary implication, as will be seen presently. These atoms, or gemmules, are thrown into the torrent of the circulation, and are consequently carried to and fro throughout the body; supplied with proper nutriment, they multiply by self-division. In this condition of gemmules they may remain and be transmitted from parent to offspring through several generations, until, in the course of their wanderings, they meet with other gemmules, and then—as in the union of germ-cell with sperm-cell—a conjugation takes place, and a cell is developed precisely similar to the original progenitor. Not only are gemmules supposed to be thrown off by every cell in the organism, but by every cell in every stage of its development, so that the gemmules circulating through the body at any given moment represent all the modifications which the parent cells have undergone; thus, if a cell becomes diseased it throws off diseased gemmules, which mingle with the earlier and healthier gemmules already thrown off. But neither the diseased nor the healthy gemmule will multiply and develop unless it meets with a fitting nidus and a fitting cell. The hypothesis further assumes that these gemmules have such a mutual affinity, that they aggregate into buds, or into sperm-cells and germ-cells, and thus reproduce the whole organism.

In illustration of the hypothesis Mr. Darwin adduces the case of one of the simplest Protozoa, formed of homogeneous gelatinous substance, any atom of which would represent the whole; but if the upper and lower surfaces of such an organism "were to differ in texture from the central portion, then all three parts would have to throw off atoms or gemmules, which, when aggregated by mutual

affinity, would form either buds or the sexual elements. Precisely the same view may be extended to one of the higher animals; in this case many thousand gemmules must be thrown off from the various parts of the body. Now when the leg, for instance, of a salamander is cut off, a slight crust forms over the wound, and beneath this crust the uninjured cells, or units of bone, muscle, nerves, &c, are supposed to unite with the diffused gemmules of those cells, which, in the perfect leg, come next in order; and these as they become slightly developed unite with others, and so on, until a papilla of soft cellular tissue, the budding leg, is formed, and in time a perfect leg. Thus that portion of the leg which had been cut off, neither more nor less, would be reproduced. If the tail or leg of a young animal had been cut off, a young tail only would have been reproduced, for gemmules of all the units which compose the tail are diffused throughout the body at all ages. But during the adult state the gemmules of the larval tail would remain dormant, for they would not meet with pre-existing cells in a proper state of development with which to unite. If from changed conditions any part of the body should become modified, the gemmules, which are merely minute portions of the contents of the cells forming the part, would naturally reproduce the same modification. But gemmules previously derived from the same part before it had undergone any change would still be diffused throughout the organism, and could be transmitted from generation to generation; so that under favourable circumstances they would be redeveloped, and then the new modification would be for a time or for ever lost."

Mr. Darwin gives a brief indication of four hypotheses, which more or less resemble the one proposed by him. I presume that a little research might extend the four to forty, all differing but slightly in their terms, and all equally vague. It is certain that one of these is two thousand years old, for Aristotle combats it. He tells us, in his extremely remarkable treatise on Generation, that there were in his day teachers who affirmed that if the whole organism arises from a First Principle (which in modern theories would be called the Cell), each part of that whole must also have its First Principle, "and hence if there is a sperm for the whole, there must likewise be a sperm for each particular part. Children are observed to resemble their parents not only in congenital peculiarities, but also in those which are subsequently acquired: they inherit even scars." We thus see that not only was the hypothesis of gemmules (principles) suggested, but it was intended to explain the very phenomena of inheritance which mainly arrest Mr. Darwin. It is unnecessary here to reproduce Aristotle's exposition and refutation of the hypothesis (I have done so elsewhere1); nor is it worth while to hunt up the

various imitations of it in the writings of subsequent physiologists; enough will be done if an illustration be given of the vagueness which rendered it infertile even in the mind of the illustrious Haller, who also assumed the constant presence of dormant germs or gemmules, and by them explained the regeneration of amputated limbs. "The head contains germs of the tail, and the tail germs of the head, so that when the one is cut off the other furnishes a supply of germs, and these consequently receiving more nutriment, are developed."1 That which distinguishes Mr. Darwin's hypothesis from all its predecessors is the precision with which it fixes what was vague and shifting, by assigning the several conditions that are necessary. In this respect it is analogous to his hypothesis of Natural Selection, which gave a sudden illumination to the old doctrine of Evolution, by substituting a precise and verifiable conception for the vague or metaphysical conceptions which were current.

Pangenesis has not the advantages of Natural Selection, and cannot therefore hope for so ready an acceptance. It has the disadvantage of not being readily grasped, nor easily brought into confrontation with facts. It has the still greater disadvantage of being hypothetical throughout: not being one supposition put forward to harmonise a series of facts, but a series of suppositions, every one of which needs proof.

A glance at the hypothesis of Natural Selection will discover that it is simply the introduction of one inference in the series of facts. That organic forms vary is a fact; that they are variable under adaptive conditions is a fact; that the Struggle for Existence necessarily brings about such conditions, and causes organic forms to vary, is also a fact; and the sole part played by pure inference in the construction of the hypothesis is the inferring that what is proved to be true in many cases is also true in all; that the Struggle for Existence, which demonstrably causes various changes, has in the long course of ages produced all the divergences of organic form. But a very different aspect is presented by Pangenesis: all its elements are inferences; not one of them can be admitted as proven. That cells throw off gemmules is an inference; what Science pretends to know respecting cell-generation is, that cells arise out of a plasma, and multiply by self-division. Some physiologists deny that cells ever arise except by generation from some pre-existing cell; but no one has attempted to prove that this generation takes place by means of gemmules. A second inference, and one still more remote, is that every part of the cell throws off its gemmules, and does so at every stage of its existence. A third inference is, that the gemmules are carried along in the circulation without undergoing modification or

decomposition. A fourth inference is, that the gemmules multiply by self-division; a fifth, that they develop only on meeting with cells which precede them in the regular order of growth; a sixth, that they are transmitted from generation to generation in the dormant state; and the seventh inference is, that they have a mutual affinity, which causes them to aggregate into sexual elements. The hypothesis is thus seen to be one wholly constructed out of suppositions, each and all of which may be erroneous, every one of them being necessary to the integrity of the scheme. This seems to me the most serious objection to be urged against it; and on this ground many who welcomed Natural Selection as a flash of light will be timid in their acceptance of Pangenesis, lest it prove a will-o'-wisp.

And yet even should it prove a will-o'-wisp it is worth our following, if we follow circumspectly, for it hovers over lands where we may find valuable material. As an hypothesis it so links together wide classes of facts that it may be a clue to great discoveries. The one danger attending our acceptance of it is the danger which attends all hypothesis, that of our forgetting its real nature, when instead of using it as a provisional mode of grouping together unexplained facts, we use it as an explanation, and thus either indolently cease further search, or fabricate fresh errors by deductions from the hypothesis as from an established datum.

One objection, and only one, can find a place in these concluding paragraphs. We confess that it is a mystery which has baffled science to explain how a complex organism can reproduce its likeness, transmitting minute particularities and acquired characters; and Mr. Darwin's readers may object that this mystery is only shifted, not cleared up, by the gemmules; since, inasmuch as the cell is an aggregate of atoms and contains differentiated parts, some cells indeed being more complex in organisation than many animals and plants, to endow this cell with the power of reproducing its likeness by means of a gemmule is to endow it with that very power which was pronounced mysterious in the larger organism. And here we may recall the point mentioned in our exposition of the hypothesis, namely, that every part of the cell must throw off its separate gemmule. The cell being an aggregate of physical, chemical, and structural characters, and its specific nature being determined by this aggregation, obviously no one portion of it would suffice for the reproduction of the whole—the nucleus, the protoplasm, the cell-wall, the cell-processes must each throw off its separate gemmule.

This consideration seems at first to throw a great obstacle in the way of the hypothesis, yet it may on closer scrutiny turn out one of its most steadfast pillars. The objection is that if every

part of the cell gives off its gemmules, the reproduction of the whole cell with all its structural relations is just as incomprehensible as the reproduction of the whole organism with all its structural relations, and that the mystery is only shifted from the organism to the cell; the gemmule is only the old germ or egg in a new guise. The mystery, I grant, is shifted, not cleared up; yet in this process of shifting we may be approaching the solution. That the cells throw off gemmules we admitted to be hypothetical, yet there seems ample justification for the assumption, if we regard the germ as only the gemmule "writ large." We know, beyond dispute, that the minute germ, which requires the microscope for examination, does contain within itself the inherited characters of the parent organism—in its seemingly simple substance lie embedded the forces which will develop into an organism of colossal size and complex structure, the lines of direction to be taken by these forces being so predetermined that the germ, which is a mere cell, contains potentially the Roman nose or the supplementary finger, the gouty disposition or the musical susceptibility, which distinguished the organism whence that germ issued. If, therefore, an organism is known to be represented in its germ, if all its parts with all their special peculiarities are in miniature in the germ, there can be no valid ground for denying that a cell may be represented in a gemmule of measureless minuteness. The consequence of this admission is, that as the germ is an aggregate representing all the parts of the organism, so the gemmule is an aggregate representing all the parts of the cell. It may, perhaps, be desirable to have separate names to designate the successive stages of representation: the cell-gemmules, which represent the cell-parts, and which, when aggregated into cell-germs, represent the entire cell; and these cell-germs aggregated into ovules represent the entire organism; but this is only a question of naming, and our attention should be more directly fixed on the advantage which accrues from the hypothesis having shifted the ground from a consideration of the reproduction of the organism to the reproduction of each separate cell. It brings us nearer to the elements. Physiologists are too apt to forget, when they speak of the organism reproducing itself as a Whole, that this Whole is only a subjective conception which summarises the Parts, and that in point of fact it is the Parts which are reproduced. Mr. Darwin's hypothesis fixes attention on the elements, and his view of the way these are reproduced harmonises with the way in which we conceive the organism to be reproduced. His gemmules are the origin of our old acquaintances the germs.